MKS INSTRUMENTS INC
S-1/A, 1999-03-23
INDUSTRIAL INSTRUMENTS FOR MEASUREMENT, DISPLAY, AND CONTROL
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<PAGE>   1
 
   
     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON MARCH 23, 1999
    
                                                      REGISTRATION NO. 333-71363
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------
 
   
                                AMENDMENT NO. 3
    
                                       TO
 
                                    FORM S-1
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                            ------------------------
 
                             MKS INSTRUMENTS, INC.
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)
 
<TABLE>
<S>                                    <C>                                    <C>
            MASSACHUSETTS                               3823                                04-2277512
   (STATE OR OTHER JURISDICTION OF          (PRIMARY STANDARD INDUSTRIAL                 (I.R.S. EMPLOYER
    INCORPORATION OR ORGANIZATION)          CLASSIFICATION CODE NUMBER)               IDENTIFICATION NUMBER)
</TABLE>
 
                               SIX SHATTUCK ROAD
                               ANDOVER, MA 01810
                                 (978) 975-2350
              (ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER,
       INCLUDING AREA CODE, OF REGISTRANT'S PRINCIPAL EXECUTIVE OFFICES)
                            ------------------------
 
                                JOHN R. BERTUCCI
                CHAIRMAN, CHIEF EXECUTIVE OFFICER, AND PRESIDENT
                             MKS INSTRUMENTS, INC.
                               SIX SHATTUCK ROAD
                               ANDOVER, MA 01810
                                 (978) 975-2350
               (NAME, ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE
               NUMBER, INCLUDING AREA CODE, OF AGENT FOR SERVICE)
                            ------------------------
 
                                   COPIES TO:
 
<TABLE>
<S>                                                 <C>
               MARK G. BORDEN, ESQ.                                DAVID C. CHAPIN, ESQ.
                 HALE AND DORR LLP                                     ROPES & GRAY
                  60 STATE STREET                                 ONE INTERNATIONAL PLACE
            BOSTON, MASSACHUSETTS 02109                         BOSTON, MASSACHUSETTS 02110
                  (617) 526-6000                                      (617) 951-7000
</TABLE>
 
                            ------------------------
 
    APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: As soon as
practicable after the effective date hereof.
 
    If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, check the following box.  [ ]
 
    If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, check the following box and
list the Securities Act registration statement number of the earlier effective
registration statement for the same offering.  [ ]
 
    If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering.  [ ]
 
    If this Form is a post-effective amendment filed pursuant to Rule 462(d)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering.  [ ]
 
    If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box:  [ ]
 
    THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SECTION 8(a), MAY
DETERMINE.
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>   2
 
The information contained in this prospectus is not complete and may be changed.
The underwriters may not confirm sales of these securities until the
registration statement filed with the Securities and Exchange Commission becomes
effective. This prospectus is not an offer to sell these securities, and is not
soliciting an offer to buy these securities in any state where the offer or sale
is not permitted.
 
   
                  SUBJECT TO COMPLETION, DATED MARCH 23, 1999
    
 
                                6,500,000 SHARES
                                   [MKS LOGO]
 
                                  COMMON STOCK
 
     MKS Instruments, Inc. is offering 6,000,000 shares of its common stock and
the selling stockholders are selling an additional 500,000 shares. This is MKS's
initial public offering and no public market currently exists for its shares. We
have been approved for quotation on the Nasdaq National Market under the symbol
"MKSI" for the shares we are offering. We estimate that the initial public
offering price will be between $15.00 and $17.00.
 
                            ------------------------
 
     INVESTING IN OUR COMMON STOCK INVOLVES A HIGH DEGREE OF RISK. SEE "RISK
FACTORS" BEGINNING ON PAGE 7.
 
                            ------------------------
 
<TABLE>
<CAPTION>
                                                              Per Share    Total
                                                              ---------    -----
<S>                                                           <C>          <C>
Public Offering Price                                             $          $
Discounts and Commissions to Underwriters                         $          $
Proceeds to MKS                                                   $          $
Proceeds to the Selling Stockholders                              $          $
</TABLE>
 
     NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR DETERMINED IF THIS
PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
     MKS has granted the underwriters a 30-day option to purchase up to an
additional 975,000 shares of common stock to cover over-allotments.
 
                            ------------------------
 
NATIONSBANC MONTGOMERY SECURITIES LLC
                          DONALDSON, LUFKIN & JENRETTE
                                                                 LEHMAN BROTHERS
 
               The date of this prospectus is             , 1999
<PAGE>   3
                              MKS INSTRUMENTS, INC.
                                PROSPECTUS COVER
                                  MARCH 2, 1999


INSIDE FRONT COVER (PG. 2):

This page is produced in four-color process. Amidst a dark background, the MKS
logo appears at the top right of the page, and to the top left is the phrase "A
Wide Range of Products Made Using MKS Process Control Instruments." Two
paragraphs describing the role MKS plays in complex advanced materials
manufacturing processes also appear on this page, and are as follows:

(first paragraph) "MKS Surrounds the Process. Technologically complex,
gas-related manufacturing processes are used to create such products as
semiconductor devices, flat panel displays, fiber optic cables, solar panels,
magnetic and optical storage media, and gas lasers. These processes build up
very thin layers of materials, step by step, through the interaction of specific
gases and materials inside tightly controlled process chambers. Maintaining
control of these complex steps throughout the entire manufacturing process is
critical to maximizing uptime, yield and throughput (second paragraph) MKS's
process control instruments are integrated into many gas-related
processes--managing the flow rates of gases entering and exiting the process
chamber; controlling the gas composition and pressure inside the chamber;
analyzing and monitoring the composition of the gases; and isolating the gases
from the outside environment."

In the center of the page is a photo montage, displaying images of semiconductor
devices, flat panel displays, fiber optic cables, solar panels, magnetic and
optical storage media and gas lasers. Each of these images has a text label
adjacent to it.

MKS, MKS Instruments, Baratron and ORION are trademarks of MKS. This prospectus
contains trademarks, service marks and trade names of companies and
organizations other than MKS.

INSIDE SPREAD (PGS. 3 AND 4):

These pages are produced in four-color process. The main focus of the spread is
the illustration of a typical process chamber, with numerous MKS products
surrounding the chamber. At the top of the illustration, centered across the two
pages, is the title "MKS Instruments...Surrounding the Process." Each product is
described in a brief paragraph, and the paragraphs appear on both sides of the
illustration--left and right columns. The paragraphs are as follows:

DIRECT LIQUID INJECTION SUBSYSTEMS
For use in the delivery of a wide variety of new materials to the process
chamber that cannot be delivered using conventional thermal-based mass flow
controllers.

AUTOMATIC PRESSURE CONTROLLERS WITH INTEGRATED BARATRON(R) PRESSURE TRANSDUCERS
A compact, integrated measurement and control package for use in controlling
upstream or downstream process chamber pressure.

ULTRA-CLEAN MASS FLOW CONTROLLERS
For the precise measurement and control of mass flow rates of inert or corrosive
gases and vapors into the process chamber.

ULTRA-CLEAN MINI-BARATRON(R) PRESSURE TRANSDUCERS
For use in gas cabinets to feed ultra-pure gases to critical process systems.

PRESSURE CONTROL VALVES
To precisely control the flow of gases to a process chamber in a wide range of
flow rates.

GAS BOX RATE OF RISE CALIBRATORS
For fast verification of mass flow controller accuracy and repeatability during
a process.

DIGITAL COLD CATHODE IONIZATION AND CONVECTION VACUUM GAUGES 
A variety of indirect pressure gauges for measuring very low chamber pressures
and conveying information digitally to host computers.

ORION(R) PROCESS MONITORS AND RESIDUAL GAS ANALYZERS
For the analysis of the composition of background and process gases inside the
process chamber.

PRESSURE SWITCHES
Provide protection of vacuum equipment and processes by signaling when
atmospheric pressure has been achieved.

BARATRON(R) PRESSURE MEASURING INSTRUMENTS
For the accurate measurement and control of a wide range of process pressures.

IN-SITU DIAGNOSTICS ACCESS VALVE
Enables accurate calibration and diagnostics of vacuum gauges and pressure
transducers while directly mounted on the process chamber.

EXHAUST THROTTLE VALVES AND AUTOMATIC PRESSURE CONTROLLERS
For isolation and downstream control of process chamber pressures and pressure
control within the exhaust systems.

HIGH VACUUM VALVES
To isolate the process chamber from both the pumps and atmospheric gases.

HEATED PUMPING LINES
To reduce contaminants in the vacuum pump and pump exhaust stream.

VAPOR SUBLIMATION TRAP
To collect by-products and particulates that could otherwise contaminate devices
in the process chamber and damage vacuum pumps.

Prices of products shown above range from $200 to $80,000.

The above graphic depicts a generalized process chamber with a number of MKS's 
manufactured products shown.
<PAGE>   4
 
     YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED IN THIS PROSPECTUS. WE
HAVE NOT AUTHORIZED ANYONE TO PROVIDE YOU WITH INFORMATION DIFFERENT FROM THAT
CONTAINED IN THIS PROSPECTUS. WE ARE OFFERING TO SELL, AND SEEKING OFFERS TO
BUY, SHARES OF COMMON STOCK ONLY IN JURISDICTIONS WHERE OFFERS AND SALES ARE
PERMITTED. THE INFORMATION CONTAINED IN THIS PROSPECTUS IS ACCURATE ONLY AS OF
THE DATE OF THIS PROSPECTUS, REGARDLESS OF THE TIME OF DELIVERY OF THIS
PROSPECTUS OR OF ANY SALE OF OUR COMMON STOCK. IN THIS PROSPECTUS, "MKS," "WE,"
"US" AND "OUR" REFER TO MKS INSTRUMENTS, INC. (UNLESS THE CONTEXT OTHERWISE
REQUIRES).
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                                              PAGE
<S>                                                           <C>
Prospectus Summary..........................................    4
Risk Factors................................................    7
S Corporation and Termination of S Corporation Status.......   13
Use of Proceeds.............................................   14
Dividend Policy.............................................   14
Capitalization..............................................   15
Dilution....................................................   16
Selected Consolidated Financial Data........................   17
Management's Discussion and Analysis of Financial Condition
  and Results of Operations.................................   19
Business....................................................   28
Management..................................................   45
Certain Transactions........................................   52
Principal and Selling Stockholders..........................   53
Description of Capital Stock................................   54
Shares Eligible for Future Sale.............................   56
Underwriting................................................   57
Legal Matters...............................................   58
Experts.....................................................   58
Additional Information......................................   59
Index to Consolidated Financial Statements..................  F-1
</TABLE>
 
                                        3
<PAGE>   5
 
                               PROSPECTUS SUMMARY
 
     This summary highlights information contained elsewhere in this prospectus.
You should read this entire prospectus carefully. Unless otherwise indicated,
all information contained in this prospectus assumes that the underwriters will
not exercise their over-allotment option. This prospectus contains forward-
looking statements, which involve risks and uncertainties. MKS's actual results
could differ materially from those anticipated in these forward-looking
statements as a result of certain factors, including those set forth under "Risk
Factors" and elsewhere in this prospectus. All information contained in this
prospectus reflects an amendment to MKS's Articles of Organization to be
effected prior to the closing of this offering to convert the shares of Class A
common stock and Class B common stock into a single class of common stock.
 
                             MKS INSTRUMENTS, INC.
 
     We are a leading worldwide developer, manufacturer and supplier of
instruments and components used to measure, control and analyze gases in
semiconductor manufacturing and similar industrial manufacturing processes. We
sold products to over 4,000 customers in 1998. In addition to semiconductors,
our products are used in processes to manufacture a diverse range of products,
such as flat panel displays, solar cells, gas lasers, fiber optic cables,
diamond thin films and coatings for food packagings.
 
     The ability of semiconductor device manufacturers to offer integrated
circuits with smaller geometries and greater functionality at higher speeds
requires continuous improvements in semiconductor process equipment and process
controls. Manufacturing a semiconductor, or a similar industrial product,
requires hundreds of process steps, many of which involve the precise
measurement and control of gases. In the fabrication of semiconductors, for
example, these process steps take place within a process chamber. Specific gas
mixtures at precisely controlled pressures are used in the process chamber to
control the required process atmosphere and are used as a source of material to
manufacture a semiconductor.
 
     Given the complexity of the semiconductor manufacturing process, the value
of the products manufactured and the significant cost of semiconductor
manufacturing equipment and facilities, significant importance is placed upon:
 
     - uptime, which is the amount of time that semiconductor manufacturing
       equipment is available for processing
 
     - yield, which is the ratio of acceptable output to total output
 
     - throughput, which is the aggregate output that can be processed per hour
 
     The design and performance of instruments that control the pressure or flow
of gases are becoming more critical to the semiconductor manufacturing process
since they directly affect uptime, yield and throughput. In addition, the
increasing sophistication of semiconductor devices requires an increase in the
number of components and subsystems used in the design of semiconductor
manufacturing process tools. To address manufacturing complexity, improve
quality and reliability, and ensure long-term service and support, semiconductor
device manufacturers and semiconductor capital equipment manufacturers are
increasingly seeking to reduce their supplier base and are, therefore, choosing
to work with suppliers that provide a broad range of integrated, technologically
advanced products backed by worldwide service and support.
 
                                        4
<PAGE>   6
 
     We believe that we offer the widest range of pressure and vacuum
measurement and control products serving the semiconductor industry. Our
products measure pressures from as low as one trillionth of atmospheric pressure
to as high as two hundred times atmospheric pressure. Our objective is to be the
leading worldwide supplier of instruments and components used to measure,
control and analyze gases in semiconductor and other advanced thin-film
processing applications and to help semiconductor device manufacturers achieve
improvements in their return on investment capital. Our strategy to accomplish
this objective includes:
 
     - extending our technology leadership
 
     - continuing to broaden our comprehensive product offering
 
     - building upon our close working relationships with customers
 
     - expanding the application of our existing technologies to related markets
 
     - leveraging our global infrastructure and world class manufacturing
       capabilities
 
     For over 25 years, we have focused on satisfying the needs of semiconductor
capital equipment manufacturers and semiconductor device manufacturers. As a
result, we have established long-term relationships with many of our customers.
We sell our products primarily to:
 
     - semiconductor capital equipment manufacturers
 
     - semiconductor device manufacturers
 
     - industrial manufacturing companies
 
     - university, government and industrial research laboratories
 
     Our customers include Applied Materials, Inc., Lam Research Corporation,
Novellus Systems, Inc., Tokyo Electron Limited, Inc., Air Products and
Chemicals, Inc. and Motorola, Inc. We sell our products primarily through our
direct sales force located in 22 offices worldwide.
 
     MKS Instruments, Inc. is a Massachusetts corporation organized in June
1961. Our principal executive offices are located at Six Shattuck Road, Andover,
MA 01810, and our telephone number is (978) 975-2350.
 
                                        5
<PAGE>   7
 
                                  THE OFFERING
 
Common stock offered by MKS...........  6,000,000 shares
 
Common stock offered by the selling
stockholders..........................  500,000 shares
 
Common stock to be outstanding after
this offering.........................  24,053,167 shares
 
Use of proceeds.......................  For distributions to current
                                        stockholders and general corporate
                                        purposes. See "Use of Proceeds" and "S
                                        Corporation and Termination of S
                                        Corporation Status."
 
Nasdaq National Market symbol.........  MKSI
 
     The common stock to be outstanding after this offering is based on shares
outstanding as of December 31, 1998 and excludes 2,132,575 shares of common
stock issuable upon the exercise of options outstanding as of such date at a
weighted average exercise price of $5.19 per share. See "Capitalization" and
Note 8 of Notes to Consolidated Financial Statements.
 
                      SUMMARY CONSOLIDATED FINANCIAL DATA
 
     MKS has been treated as an S corporation for federal income tax purposes
since July 1, 1987. As an S corporation, MKS has not been subject to federal,
and certain state, income taxes. The pro forma net income reflects the provision
for income taxes that would have been recorded had MKS been a C corporation,
assuming an effective tax rate of 39.0% for 1994 and 1995, and 38.0% for 1996,
1997 and 1998. As a result of terminating its S corporation status upon the
closing of this offering, MKS will record a one-time non-cash credit to
historical earnings for additional deferred taxes. If this credit to earnings
had occurred at December 31, 1998, the amount would have been approximately $3.9
million. This amount is expected to increase through the closing of this
offering and is excluded from pro forma net income. See Notes 2 and 9 of Notes
to Consolidated Financial Statements.
 
   
     Pro forma balance sheet data set forth below reflects the liability for the
distribution of an estimated $35.9 million, calculated as of December 31, 1998,
of cumulative undistributed S corporation taxable income for which stockholders
of record on the day prior to the effective date of the registration statement
have been or will be taxed. The pro forma net income per share for 1998 set
forth below reflects the effect of an assumed issuance of sufficient shares to
fund this distribution as of January 1, 1998. The distribution will be made out
of the proceeds of this offering. The actual amount to be distributed is
expected to be $40.0 million which is the estimated balance of the accumulated
adjustments account as of the day prior to the closing of this offering, subject
to adjustments. See "S Corporation and Termination of S Corporation Status." The
pro forma as adjusted balance sheet data reflects the sale of 6,000,000 shares
of common stock at an assumed initial public offering price of $16.00 per share,
after deducting the estimated underwriting discount and offering expenses
payable by MKS. THE HISTORICAL NET INCOME PER SHARE DATA SET FORTH BELOW DOES
NOT INCLUDE PROVISIONS FOR FEDERAL INCOME TAXES BECAUSE PRIOR TO THE CLOSING OF
THIS OFFERING, MKS WAS TREATED AS AN S CORPORATION FOR FEDERAL INCOME TAX
PURPOSES.
    
 
   
<TABLE>
<CAPTION>
                                                                              YEAR ENDED DECEMBER 31,
                                                              --------------------------------------------------------
                                                                1994        1995        1996        1997        1998
                                                              --------    --------    --------    --------    --------
                                                                       (IN THOUSANDS, EXCEPT PER SHARE DATA)
<S>                                                           <C>         <C>         <C>         <C>         <C>
STATEMENT OF INCOME DATA:
Net sales...................................................  $106,829    $157,164    $170,862    $188,080    $139,763
Gross profit................................................    47,016      69,461      68,854      80,474      55,979
Income from operations......................................    12,087      24,106      16,068      23,963       9,135
Net income..................................................  $ 10,003    $ 21,658    $ 12,503    $ 20,290    $  7,186
HISTORICAL NET INCOME PER SHARE:
  Basic.....................................................  $   0.55    $   1.20    $   0.69    $   1.12    $   0.40
                                                              ========    ========    ========    ========    ========
  Diluted...................................................  $   0.55    $   1.20    $   0.69    $   1.10    $   0.38
                                                              ========    ========    ========    ========    ========
PRO FORMA STATEMENT OF INCOME DATA(1):
Pro forma net income........................................  $  6,590    $ 13,821    $  8,248    $ 13,806    $  5,044
Pro forma net income per share:
  Basic.....................................................  $   0.37    $   0.77    $   0.46    $   0.76    $   0.25
                                                              ========    ========    ========    ========    ========
  Diluted...................................................  $   0.37    $   0.77    $   0.46    $   0.76    $   0.24
                                                              ========    ========    ========    ========    ========
</TABLE>
    
 
<TABLE>
<CAPTION>
                                                                       DECEMBER 31, 1998
                                                              ------------------------------------
                                                                                        PRO FORMA
                                                               ACTUAL     PRO FORMA    AS ADJUSTED
                                                              --------    ---------    -----------
                                                                         (IN THOUSANDS)
<S>                                                           <C>         <C>          <C>
BALANCE SHEET DATA:
Cash and cash equivalents...................................  $ 11,188    $ 11,188      $ 63,942
Working capital (deficit)...................................    31,493      (4,433)       84,247
Total assets................................................    96,232      96,232       148,986
Short-term obligations......................................    12,819      12,819        12,819
Long-term obligations, less current portion.................    13,786      13,786        13,786
Stockholders' equity........................................    54,826      18,900       107,580
</TABLE>
 
- ---------------
(1) Data is computed on the same basis as Note 2 of Notes to Consolidated
    Financial Statements.
 
                                        6
<PAGE>   8
 
                                  RISK FACTORS
 
     You should consider carefully the risks described below before you decide
to buy our common stock. If any of the following risks actually occur, our
business, financial condition or results of operations would likely suffer. In
such case, the trading price of our common stock could fall, and you may lose
all or part of the money you paid to buy our common stock.
 
     This prospectus contains forward-looking statements that involve risks and
uncertainties. These forward-looking statements are usually accompanied by words
such as "believes," "anticipates," "plans," "expects" and similar expressions.
Our actual results may differ materially from the results discussed in the
forward-looking statements because of factors such as the Risk Factors discussed
below.
 
   
OUR BUSINESS DEPENDS SUBSTANTIALLY ON SEMICONDUCTOR INDUSTRY CAPITAL SPENDING
WHICH IS CHARACTERIZED BY PERIODIC FLUCTUATIONS THAT MAY CAUSE A REDUCTION IN
DEMAND FOR OUR PRODUCTS
    
 
     We estimate that approximately 60% of our sales during 1997 and 1998 were
to semiconductor capital equipment manufacturers and semiconductor device
manufacturers, and we expect that sales to such customers will continue to
account for a substantial majority of our sales. Our business depends
substantially upon the capital expenditures of semiconductor device
manufacturers, which in turn depend upon the demand for semiconductors and other
products utilizing semiconductors. Periodic reductions in demand for the
products manufactured by semiconductor capital equipment manufacturers and
semiconductor device manufacturers may adversely affect our business, financial
condition and results of operations. Historically, the semiconductor market has
been highly cyclical and has experienced periods of overcapacity, resulting in
significantly reduced demand for capital equipment. For example, in 1996 and
1998 the semiconductor industry experienced a significant decline, which caused
a number of our customers to reduce their orders. We cannot be certain that the
current semiconductor downturn that began in 1998 will not continue. A further
decline in the level of orders as a result of any future downturn or slowdown in
the semiconductor industry could have a material adverse effect on our business,
financial condition and results of operations.
 
   
     WE DERIVE A SIGNIFICANT PORTION OF OUR REVENUE FROM THE ASIAN MARKETS, AS
DO OUR SEMICONDUCTOR CAPITAL EQUIPMENT CUSTOMERS, AND THEREFORE, AN ECONOMIC
DOWNTURN IN THE ASIAN MARKETS WOULD LIKELY REDUCE DEMAND FOR OUR PRODUCTS
    
 
     The financial markets in Asia, one of our principal international markets,
have experienced significant turbulence. Turbulence in the Asian markets can
adversely affect our net sales and results of operations. Our direct net sales
to customers in Asian markets have been approximately 17% to 18% of total net
sales for the past three years. Our sales include both direct sales to the
semiconductor industry in Asia, as well as to semiconductor capital equipment
manufacturers that derive a significant portion of their revenue from sales to
the Asian semiconductor industry. Turbulence in the Asian markets began to
adversely affect the semiconductor device manufacturers and semiconductor
capital equipment manufacturers in the fourth quarter of 1997 and continued to
adversely affect them in 1998. We expect the turbulence in the Asian markets
will continue to adversely affect sales of semiconductor capital equipment
manufacturers for at least the first quarter of 1999. As a result, for at least
the first quarter we currently expect that our 1999 quarterly net sales and net
income will be less than net sales and net income for the comparable quarter of
1998.
 
   
OUR QUARTERLY NET SALES ARE BASED ON SHIPMENTS MADE SHORTLY AFTER CUSTOMER
ORDERS ARE PLACED AND THEREFORE, FLUCTUATIONS IN DEMAND WITHIN A QUARTER WILL
CAUSE A FLUCTUATION IN THAT QUARTER'S NET SALES
    
 
     A substantial portion of our shipments occur shortly after an order is
received and therefore we operate with a low level of backlog. As a consequence
of the just-in-time nature of shipments and the low level of backlog, a decrease
in demand for our products from one or more customers could occur with limited
advance notice and could have a material adverse effect on our results of
operations in any particular period.
 
                                        7
<PAGE>   9
 
   
DUE TO OUR FIXED COSTS WE MAY BE UNABLE TO ADJUST SPENDING QUICKLY ENOUGH TO
COMPENSATE FOR SHORTFALLS IN NET SALES WHICH MAY LEAD TO REDUCED OPERATING
RESULTS IF OUR NET SALES ARE BELOW EXPECTATIONS
    
 
     A significant percentage of our expenses are relatively fixed and based in
part on expectations of future net sales. The inability to adjust spending
quickly enough to compensate for any shortfall would magnify the adverse impact
of a shortfall in net sales on our results of operations. Factors that could
cause fluctuations in our net sales include:
 
- - the timing of the receipt of orders from major customers
 
- - shipment delays
 
- - disruption in sources of supply
 
- - seasonal variations of capital spending by customers
 
- - production capacity constraints
 
- - specific features requested by customers
 
     For example, we were in the process of increasing production capacity when
the semiconductor capital equipment market began to experience a significant
downturn in 1996. This downturn had a material adverse effect on our operating
results in the second half of 1996 and the first half of 1997. After an increase
in business in the latter half of 1997, the market experienced another downturn
in 1998, which had a material adverse effect on our 1998 operating results. As a
result of the factors discussed above, it is likely that we will in the future
experience quarterly or annual fluctuations and that, in one or more future
quarters, our operating results will fall below the expectations of public
market analysts or investors. In any such event, the price of our common stock
could decline significantly.
 
   
THE LOSS OF NET SALES TO ANY ONE OF OUR MAJOR CUSTOMERS WOULD LIKELY HAVE A
MATERIAL ADVERSE EFFECT ON US
    
 
     Our five largest customers in 1996, 1997 and 1998 accounted for
approximately 26%, 32% and 24%, respectively, of our net sales. The loss of a
major customer or any reduction in orders by such customers, including
reductions due to market or competitive conditions, would likely have a material
adverse effect on our business, financial condition and results of operations.
During 1998, one customer, Applied Materials, Inc., accounted for approximately
16% of our net sales. While we have entered into a purchase contract with
Applied Materials, Inc. which expires in 2000 unless it is extended by mutual
agreement, none of our significant customers, including Applied Materials, Inc.,
has entered into an agreement requiring it to purchase any minimum quantity of
our products. The demand for our products from our semiconductor capital
equipment customers depends in part on orders received by them from their
semiconductor device manufacturer customers.
 
     Attempts to lessen the adverse effect of any loss or reduction through the
rapid addition of new customers could be difficult because prospective customers
typically require lengthy qualification periods prior to placing volume orders
with a new supplier. Our future success will continue to depend upon:
 
     - our ability to maintain relationships with existing key customers
 
     - our ability to attract new customers
 
     - the success of our customers in creating demand for their capital
       equipment products which incorporate our products
 
   
OUR INABILITY TO CONVINCE SEMICONDUCTOR DEVICE MANUFACTURERS TO SPECIFY THE USE
OF OUR PRODUCTS TO OUR CUSTOMERS, WHO ARE SEMICONDUCTOR CAPITAL EQUIPMENT
MANUFACTURERS, WOULD WEAKEN OUR COMPETITIVE POSITION
    
 
   
     The markets for our products are highly competitive. Our competitive
success often depends upon factors outside of our control. For example, in some
cases, particularly with respect to mass flow controllers, semiconductor device
manufacturers may direct semiconductor capital equipment manufacturers to use a
specified supplier's product in their equipment. Accordingly, for such products,
our success will depend in part on our ability to have semiconductor device
manufacturers specify that our
    
 
                                        8
<PAGE>   10
 
products be used at their semiconductor fabrication facilities. In addition, we
may encounter difficulties in changing established relationships of competitors
that already have a large installed base of products within such semiconductor
fabrication facilities.
 
   
IF OUR PRODUCTS ARE NOT DESIGNED INTO SUCCESSIVE NEW GENERATIONS OF OUR
CUSTOMERS' PRODUCTS, WE WILL LOSE SIGNIFICANT NET SALES DURING THE LIFESPAN OF
THOSE PRODUCTS
    
 
   
     New products designed by semiconductor capital equipment manufacturers
typically have a lifespan of five to ten years. Our success depends on our
products being designed into new generations of equipment for the semiconductor
industry. We must develop products that are technologically current so that they
are positioned to be chosen for use in each successive new generation of
semiconductor equipment. If our products are not chosen by our customers, our
net sales may be reduced during the lifespan of our customers' products.
    
 
   
FAILURE BY US TO IDENTIFY AND REMEDIATE ALL MATERIAL YEAR 2000 RISKS COULD CAUSE
A SIGNIFICANT DISRUPTION TO OUR BUSINESS IF WE ARE FORCED TO EXPEND SIGNIFICANT
INTERNAL RESOURCES ON YEAR 2000 REMEDIATION OR IF THE YEAR 2000 PROBLEMS OF OUR
SUPPLIERS OR CUSTOMERS CAUSE A DELAY IN SUPPLYING GOODS AND SERVICES TO US OR IN
DELAYING PAYMENT FOR PRODUCTS THAT WE HAVE SHIPPED
    
 
     We have implemented a multi-phase Year 2000 project consisting of
assessment and remediation, and testing following remediation. We cannot,
however, be certain that we have identified all of the potential risks. Failure
by us to identify and remediate all material Year 2000 risks could adversely
affect our business, financial condition and results of operations. We have
identified the following risks you should be aware of:
 
     - we cannot be certain that the entities on whom we rely for certain goods
       and services that are important for our business will be successful in
       addressing all of their software and systems problems in order to operate
       without disruption in the year 2000 and beyond
 
     - our customers or potential customers may be affected by Year 2000 issues
       that may, in part:
 
        -- cause a delay in payments for products shipped
 
        -- cause customers to expend significant resources on Year 2000
           compliance matters, rather than investing in our products
 
     - we have not developed a contingency plan related to the failure of our or
       a third-party's Year 2000 remediation efforts and may not be prepared for
       such an event
 
     Further, while we have made efforts to notify our customers who have
purchased potential non-compliant products, we cannot be sure that customers who
purchased such products will not assert claims against us alleging that such
products should have been Year 2000 compliant at the time of purchase, which
could result in costly litigation and divert management's attention.
 
   
WE INTEND TO EXPAND OUR BUSINESS OUTSIDE OF THE SEMICONDUCTOR INDUSTRY TO THE
MANUFACTURE OF, AMONG OTHER THINGS, HARD COATINGS TO MINIMIZE WEAR ON CUTTING
TOOLS, A MARKET IN WHICH WE HAVE LIMITED EXPERIENCE AND IF WE FAIL TO
SUCCESSFULLY PENETRATE SUCH MARKETS, OUR NET SALES WILL CONTINUE TO BE
VULNERABLE TO THE DOWNTURNS IN THE SEMICONDUCTOR INDUSTRY
    
 
   
     We plan to build upon our experience in manufacturing and selling gas
measurement, control and analysis products used by the semiconductor industry by
designing and selling such products for applications in other industries which
use production processes similar to those used in the semiconductor industry.
For example, we plan to expand our business to the manufacture of, among other
things, hard coatings to minimize wear on cutting tools. Any failure by us to
penetrate additional markets would limit our ability to reduce our vulnerability
to downturns in the semiconductor industry and could have a material adverse
effect on our business, financial condition and results of operations.
    
 
                                        9
<PAGE>   11
 
     We have limited experience selling our products in certain markets outside
the semiconductor industry. We cannot be certain that we will be successful in
the expansion of our business outside the semiconductor industry. Our future
success will depend in part on our ability to:
 
     - identify new applications for our products
 
     - adapt our products for such applications
 
     - market and sell such products to customers
 
   
THE SEMICONDUCTOR INDUSTRY IS SUBJECT TO RAPID DEMAND SHIFTS WHICH ARE DIFFICULT
TO PREDICT AND AS A RESULT, OUR INABILITY TO EXPAND OUR MANUFACTURING CAPACITY
IN RESPONSE TO THESE RAPID SHIFTS MAY CAUSE A REDUCTION IN OUR MARKET SHARE
    
 
     During 1999, we plan to add manufacturing capacity to our Austin, Texas
operations and further equip our cleanroom facilities in Andover and Methuen,
Massachusetts. Our ability to increase sales of certain products depends in part
upon our ability to expand our manufacturing capacity for such products in a
timely manner. If we are unable to expand our manufacturing capacity on a timely
basis or to manage such expansion effectively, our customers could seek such
products from others and our market share could be reduced. Because the
semiconductor industry is subject to rapid demand shifts which are difficult to
foresee, we may not be able to increase capacity quickly enough to respond to a
rapid increase in demand in the semiconductor industry. Additionally, capacity
expansion could increase our fixed operating expenses and if sales levels do not
increase to offset the additional expense levels associated with any such
expansion, our business, financial condition and results of operations could be
materially adversely affected.
 
   
SALES TO FOREIGN MARKETS CONSTITUTE APPROXIMATELY 30% OF OUR NET SALES AND,
THEREFORE, OUR NET SALES AND RESULTS OF OPERATIONS COULD BE ADVERSELY AFFECTED
BY DOWNTURNS IN ECONOMIC CONDITIONS IN COUNTRIES OUTSIDE OF THE UNITED STATES
    
 
     International sales, which include sales by our foreign subsidiaries, but
exclude direct export sales which were less than 10% of our total net sales,
accounted for approximately 30% of net sales in 1996, 27% of net sales in 1997
and 32% of net sales in 1998. We anticipate that international sales will
continue to account for a significant portion of our net sales. In addition,
certain of our key domestic customers derive a significant portion of their
revenues from sales in international markets. Therefore, our sales and results
of operations could be adversely affected by economic slowdowns and other risks
associated with international sales.
 
   
UNFAVORABLE EXCHANGE RATE FLUCTUATIONS MAY LEAD TO LOWER GROSS MARGINS OR MAY
CAUSE US TO RAISE PRICES WHICH COULD RESULT IN REDUCED SALES
    
 
     Exchange rate fluctuations could have an adverse effect on our net sales
and results of operations and we could experience losses with respect to our
hedging activities. Unfavorable currency fluctuations could require us to
increase prices to foreign customers which could result in lower net sales by us
to such customers. Alternatively, if we do not adjust the prices for our
products in response to unfavorable currency fluctuations, our results of
operations could be adversely affected. In addition, sales made by our foreign
subsidiaries are denominated in the currency of the country in which these
products are sold and the currency we receive in payment for such sales could be
less valuable at the time of receipt as a result of exchange rate fluctuations.
While we enter into forward exchange contracts and local currency purchased
options to reduce currency exposure arising from these sales and associated
intercompany purchases of inventory, we cannot be certain that our efforts will
be adequate to protect us against significant currency fluctuations or that such
efforts will not expose us to additional exchange rate risks.
 
   
COMPETITION FOR PERSONNEL IN THE SEMICONDUCTOR AND INDUSTRIAL MANUFACTURING
INDUSTRIES IS INTENSE AND BECAUSE WE DO NOT TYPICALLY HAVE EMPLOYMENT AGREEMENTS
WITH OUR EMPLOYEES, WE CANNOT BE SURE THAT WE WILL BE ABLE TO RETAIN THEM WHICH
IS AN IMPORTANT FACTOR IN ACHIEVING FUTURE SUCCESS
    
 
                                       10
<PAGE>   12
 
     Our success depends to a large extent upon the efforts and abilities of a
number of key employees and officers, particularly those with expertise in the
semiconductor manufacturing and similar industrial manufacturing industries. The
loss of key employees or officers could have a material adverse effect on our
business, financial condition and results of operations. We believe that our
future success will depend in part on our ability to attract and retain highly
skilled technical, financial, managerial and marketing personnel. Competition
for such personnel is intense, and we cannot be certain that we will be
successful in attracting and retaining such personnel. We are the beneficiary of
key-man life insurance policies on John R. Bertucci, Chairman, Chief Executive
Officer and President, in the amount of $7.2 million.
 
   
OUR PROPRIETARY TECHNOLOGY, WHICH INCLUDES 49 PATENTS AND 8 PENDING PATENT
APPLICATIONS, IS IMPORTANT TO THE CONTINUED SUCCESS OF OUR BUSINESS AND THE
FAILURE TO PROTECT THIS PROPRIETARY TECHNOLOGY MAY SIGNIFICANTLY IMPAIR OUR
COMPETITIVE POSITION
    
 
     Although we seek to protect our intellectual property rights through
patents, copyrights, trade secrets and other measures, we cannot be certain
that:
 
     - we will be able to protect our technology adequately
 
     - competitors will not be able to develop similar technology independently
 
     - any of our pending patent applications will be issued
 
     - intellectual property laws will protect our intellectual property rights
 
     - third parties will not assert that our products infringe patent,
       copyright or trade secrets of such parties
 
PROTECTION OF OUR INTELLECTUAL PROPERTY RIGHTS MAY RESULT IN COSTLY LITIGATION
 
     Litigation may be necessary in order to enforce our patents, copyrights or
other intellectual property rights, to protect our trade secrets, to determine
the validity and scope of the proprietary rights of others or to defend against
claims of infringement. Such litigation could result in substantial costs and
diversion of resources and could have a material adverse effect on our business,
financial condition and results of operations.
 
TRADING IN OUR SHARES COULD BE SUBJECT TO EXTREME PRICE FLUCTUATIONS AND YOU
COULD HAVE DIFFICULTY TRADING YOUR SHARES
 
     The market for shares in newly public technology companies is subject to
extreme price and volume fluctuations. These broad market fluctuations may
materially and adversely affect the market price of our common stock. In
addition, although our common stock will be quoted on the Nasdaq National
Market, an active trading market may not develop and be sustained after this
offering.
 
YOU WILL EXPERIENCE AN IMMEDIATE AND SUBSTANTIAL DILUTION IN THE BOOK VALUE OF
YOUR INVESTMENT
 
     Purchasers of common stock in this offering will incur immediate and
substantial dilution of $11.53 in the pro forma net tangible book value per
share of common stock from the assumed initial public offering price of $16.00
per share.
 
AFTER THIS OFFERING ONE STOCKHOLDER, ALONG WITH MEMBERS OF HIS FAMILY, WILL HAVE
CONTROLLING INTEREST IN MKS AND WILL BE ABLE TO EFFECT IMPORTANT CORPORATE
ACTIONS WITHOUT THE APPROVAL OF OTHER STOCKHOLDERS
 
     Upon consummation of this offering, John R. Bertucci, Chairman, Chief
Executive Officer and President of MKS, and members of his family will, in the
aggregate, beneficially own approximately 70% of our outstanding common stock.
As a result, these stockholders, acting together, will be able to take any of
the following actions without the approval of our public stockholders:
 
                                       11
<PAGE>   13
 
     - amend our Articles of Organization in certain respects or approve a
       merger, sale of assets or other major corporate transaction
 
     - defeat any non-negotiated takeover attempt that may be beneficial to our
       public stockholders
 
     - determine the amount and timing of dividends paid to themselves and to
       our public stockholders
 
     - otherwise control our management and operations and the outcome of all
       matters submitted for a stockholder vote, including the election of
       directors
 
CERTAIN PROVISIONS OF OUR ARTICLES OF ORGANIZATION, OUR BY-LAWS AND
MASSACHUSETTS LAW COULD DISCOURAGE POTENTIAL ACQUISITION PROPOSALS AND COULD
DELAY OR PREVENT A CHANGE IN CONTROL OF MKS
 
     Anti-takeover provisions could diminish the opportunities for stockholders
to participate in tender offers including tender offers at a price above the
then current market value of the common stock. Such provisions may also inhibit
increases in the market price of the common stock that could result from
takeover attempts. For example, while we have no present plans to issue any
preferred stock, the Board of Directors, without further stockholder approval,
may issue preferred stock that could have the effect of delaying, deterring or
preventing a change in control of MKS. The issuance of preferred stock could
adversely affect the voting power of the holders of common stock including the
loss of voting control to others. In addition, our By-Laws will provide for a
classified Board of Directors consisting of three classes. This classified board
could also have the effect of delaying, deterring or preventing a change in
control of MKS.
 
FUTURE SALES BY OUR EXISTING STOCKHOLDERS COULD ADVERSELY AFFECT THE MARKET
PRICE OF OUR COMMON STOCK
 
     Sales of our common stock in the public market following this offering
could adversely affect the market price of the common stock. All of the shares
offered under this prospectus will be freely tradable in the open market, and
 
     - 17,553,165 additional shares may be sold after the expiration of 180-day
       lock-up agreements
 
     - approximately 1,100,000 additional shares may be sold upon the exercise
       of stock options after the expiration of 180-day lock-up agreements
 
                                       12
<PAGE>   14
 
             S CORPORATION AND TERMINATION OF S CORPORATION STATUS
 
     MKS has been treated as an S corporation for federal income tax purposes
since July 1, 1987. As a result, MKS currently pays no federal, and certain
state, income tax, and all of the earnings of MKS are subject to federal, and
certain state, income taxation directly at the stockholder level. MKS's S
corporation status will terminate upon the closing of this offering, at which
time MKS will become subject to corporate income taxation under Subchapter C of
the Internal Revenue Code and applicable state income taxation law. Pro forma
statement of income data set forth in this prospectus has been adjusted to
include pro forma income tax provisions as if MKS had been a C corporation
during the relevant periods.
 
   
     As soon as practicable following the closing of this offering, MKS intends
to make a distribution to the stockholders of record on the day prior to the
effective date of the registration statement of $40.0 million, which is the
estimated amount of the "accumulated adjustments account," as of the day prior
to the closing of this offering as defined in Section 1368(a)(1) of the Internal
Revenue Code. The accumulated adjustments account is equal to the cumulative
income of MKS, as determined for federal income tax purposes, for the period MKS
was an S corporation (from July 1, 1987 through the date of the closing of this
offering) minus any distributions made to stockholders during this period. The
accumulated adjustments account for the period January 1, 1999 through the date
of the closing of this offering will equal a portion of the federal taxable
income of MKS for the entire calendar year 1999, excluding any earnings from its
international subsidiaries, determined by allocating all of the calendar year
1999 taxable income equally to each day in the year and multiplying the daily
taxable income by the number of days from January 1, 1999 through the date of
the closing of this offering. Investors purchasing shares in this offering will
not receive any portion of the distribution.
    
 
   
     MKS expects to enter into a Tax Indemnification and S Corporation
Distribution Agreement with its existing stockholders providing for, among other
things, the indemnification of MKS by such stockholders for any federal and
state income taxes, including interest and penalties, incurred by MKS if for any
reason MKS is deemed to be treated as a C corporation during any period in which
it reported its taxable income as an S corporation. The tax indemnification
obligation of each existing stockholder is limited to the aggregate amount of
all distributions made to such stockholders by MKS since July 1, 1987, minus any
taxes paid by such stockholders on such distributions plus the amount of any
refund of taxes to such stockholders as a result of such a deemed change in tax
status and is limited to each such stockholders' pro rata receipt of the
accumulated adjustments account distributions. The agreement also provides for
the payment, with interest, by the existing stockholders or MKS, as the case may
be, for the difference between the amount to be distributed and the actual
amount of accumulated adjustments account on the day immediately preceding the
closing of this offering. The actual amount of the accumulated adjustments
account on the day prior to the closing of this offering cannot be determined
until MKS calculates the amount of its taxable income for the year ending
December 31, 1999. Furthermore, the amount of the accumulated adjustments
account can be affected by income tax audits of MKS. If any audit increases or
decreases the accumulated adjustments account, MKS or the existing stockholders,
as the case may be, will also be required to make a payment with interest, of
such difference to the other party. MKS's tax returns for 1995, 1996 and 1997
are currently being audited by the Internal Revenue Service and although the
estimated accumulated adjustments account has been adjusted to reflect all
changes that MKS expects to make as a result of the audit, there can be no
assurance that additional adjustments will not be required prior to the
conclusion of the audit. Purchasers of common stock in this offering will not be
parties to the Tax Indemnification and S Corporation Distribution Agreement.
    
 
                                       13
<PAGE>   15
 
                                USE OF PROCEEDS
 
     The net proceeds we will receive from the sale of the 6,000,000 shares of
common stock offered by us are estimated to be $88,680,000 ($103,188,000 if the
underwriters' over-allotment option is exercised in full), after deducting the
estimated underwriting discount and offering expenses payable by us and assuming
an initial public offering price of $16.00 per share. We will not receive any of
the proceeds from the sale of shares by the selling stockholders.
 
   
     We will use $40.0 million of the net proceeds from this offering to pay the
stockholders of record on the day prior to the effective date of the
registration statement the estimated amount, subject to adjustment, of their
undistributed S corporation earnings as of the day prior to the closing of this
offering. See "S Corporation and Termination of S Corporation Status." We expect
to use the remainder of the net proceeds for general corporate purposes,
including working capital, product development and capital expenditures.
    
 
     A portion of the net proceeds after the S corporation distribution may also
be used for the acquisition of businesses, products and technologies that are
complementary to those of MKS. There are currently no active negotiations,
commitments or agreements with respect to any acquisition. Pending such uses, we
intend to invest the net proceeds from this offering in short-term,
investment-grade, interest-bearing securities.
 
                                DIVIDEND POLICY
 
     We currently intend, subject to our contractual obligations under the Tax
Indemnification and S Corporation Distribution Agreement, to retain earnings for
the continued development of our business. Restrictions or limitations on the
payment of dividends may be imposed in the future under the terms of credit
agreements or under other contractual provisions. In the absence of such
restrictions or limitations, the payment of any dividends will be at the
discretion of our Board of Directors.
 
                                       14
<PAGE>   16
 
                                 CAPITALIZATION
 
     The following table sets forth the capitalization of MKS (1) as of December
31, 1998, (2) on a pro forma basis to reflect distributions and adjustments in
connection with MKS's S corporation status and (3) as adjusted to reflect the
sale of 6,000,000 shares of common stock by MKS at an assumed initial public
offering price of $16.00 per share and the application of the net proceeds
therefrom. See "Use of Proceeds."
 
   
     The pro forma data reflects the liability for distribution of an estimated
$35.9 million, calculated as of December 31, 1998, of cumulative undistributed S
corporation taxable income for which stockholders of record on the day prior to
the effective date of the registration statement have been or will be taxed. The
actual amount to be distributed after the closing of this offering will be $40.0
million, which is the estimated amount of our cumulative undistributed S
corporation taxable income as of the day prior to the closing of this offering,
subject to adjustment. See "S Corporation and Termination of S Corporation
Status" and Notes 2 and 9 of Notes to Consolidated Financial Statements. The pro
forma as adjusted data have been adjusted to reflect the issuance of 6,000,000
shares of common stock at an assumed initial public offering price of $16.00 per
share, after deducting the estimated underwriting discount and offering expenses
payable by MKS. The remaining balance in retained earnings represents
accumulated earnings prior to MKS's conversion from a C corporation to an S
corporation in 1987, accumulated income in overseas subsidiaries and differences
between book and tax accumulated income.
    
 
<TABLE>
<CAPTION>
                                                                    DECEMBER 31, 1998
                                                         ---------------------------------------
                                                                                      PRO FORMA
                                                         ACTUAL       PRO FORMA      AS ADJUSTED
                                                         ------       ---------      -----------
                                                            (IN THOUSANDS, EXCEPT SHARE DATA)
<S>                                                      <C>          <C>            <C>
Long-term obligations, less current portion............  $13,786       $13,786        $ 13,786
Stockholders' equity:
  Common stock, no par value; 30,000,000 shares
     authorized, 18,053,167 shares issued and
     outstanding (actual and pro forma); 24,053,167
     shares issued and outstanding (pro forma as
     adjusted).........................................      113           113             113
  Additional paid-in capital...........................       48            48          88,728
  Retained earnings....................................   52,479        16,553          16,553
  Accumulated other comprehensive income...............    2,186         2,186           2,186
                                                         -------       -------        --------
     Total stockholders' equity........................   54,826        18,900         107,580
                                                         -------       -------        --------
          Total capitalization.........................  $68,612       $32,686        $121,366
                                                         =======       =======        ========
</TABLE>
 
     The common stock to be outstanding after this offering is based on shares
outstanding as of December 31, 1998 and excludes 2,132,575 shares of common
stock issuable upon the exercise of options outstanding as of such date at a
weighted average exercise price of $5.19 per share. See Note 8 of Notes to
Consolidated Financial Statements.
 
                                       15
<PAGE>   17
 
                                    DILUTION
 
   
     As of December 31, 1998, MKS had a net tangible book value of $54,826,000,
or $3.04 per share of common stock. After taking into account the sale of the
shares offered hereby by MKS, the pro forma net tangible book value as of
December 31, 1998 would have been $107,580,000, or $4.47 per share. The pro
forma net tangible book value assumes that the proceeds to MKS, net of offering
expenses and commissions, will be approximately $52,754,000. This number has
also been adjusted to take into account the distribution to stockholders of
record on the day prior to the effective date of the registration statement of
the accumulated undistributed S corporation taxable income for which such
taxpayers have been or will be taxed as of December 31, 1998. That amount is
estimated to be $35.9 million as of December 31, 1998. No other changes
occurring after December 31, 1998 have been taken into account. Based on the
foregoing, there would be an immediate increase in net tangible book value to
existing stockholders attributable to new investors of $2.92 per share and the
immediate dilution of $11.53 per share to new investors. The following table
illustrates this per share dilution:
    
 
<TABLE>
<S>                                                           <C>       <C>
Assumed initial public offering price per share.............            $16.00
  Net tangible book value per share at December 31, 1998....  $ 3.04
  Decrease per share attributable to the S corporation
     distribution...........................................   (1.49)
  Increase per share attributable to new investors..........    2.92
                                                              ------
Pro forma net tangible book value per share after this
  offering..................................................              4.47
                                                                        ------
Dilution per share to new investors.........................            $11.53
                                                                        ======
</TABLE>
 
     The following table sets forth, on a pro forma basis as of December 31,
1998, (1) the number of shares of common stock purchased from MKS, (2) the total
consideration paid to MKS and (3) the average price paid per share by existing
stockholders and by the new investors purchasing shares of common stock in this
offering, at an assumed initial public offering price of $16.00 per share.
Underwriting discounts, commissions and other estimated offering expenses have
not been deducted. Shares owned by existing stockholders will be reduced by the
number of shares sold by them in this offering.
 
<TABLE>
<CAPTION>
                                      SHARES PURCHASED         TOTAL CONSIDERATION
                                    ---------------------    -----------------------    AVERAGE PRICE
                                      NUMBER      PERCENT       AMOUNT       PERCENT      PER SHARE
                                    ----------    -------    ------------    -------    -------------
<S>                                 <C>           <C>        <C>             <C>        <C>
Existing stockholders.............  18,053,167      75.1%    $    161,000       0.2%       $0.009
New investors.....................   6,000,000      24.9       96,000,000      99.8        $16.00
                                    ----------     -----     ------------     -----
          Total...................  24,053,167     100.0%      96,161,000     100.0%
                                    ==========     =====     ============     =====
</TABLE>
 
     As of December 31, 1998, there were options outstanding to purchase a total
of 2,132,575 shares of common stock, at a weighted average exercise price of
$5.19 per share and 2,401,793 additional shares reserved for future grants of
issuances under MKS's stock option and stock purchase plans. To the extent that
any of these options are exercised, there will be further dilution to new
investors.
 
                                       16
<PAGE>   18
 
                      SELECTED CONSOLIDATED FINANCIAL DATA
 
     The following selected financial data as of December 31, 1997 and 1998 and
for the years ended December 31, 1996, 1997 and 1998 have been derived from
MKS's financial statements, included elsewhere in this prospectus, which have
been audited by PricewaterhouseCoopers LLP, independent accountants, as
indicated in their report. The selected financial data as of December 31, 1994,
1995 and 1996 and for the years ended December 31, 1994 and 1995 are derived
from financial statements, which were also audited by PricewaterhouseCoopers
LLP, not included herein. The data should be read in conjunction with the
Consolidated Financial Statements, including the Notes thereto, and with
"Management's Discussion and Analysis of Financial Condition and Results of
Operations" included elsewhere in this prospectus.
 
   
     MKS has been treated as an S corporation under the applicable provisions of
the Internal Revenue Code since July 1, 1987. As an S corporation, MKS has not
been subject to federal, and certain state, income taxes. The pro forma net
income set forth below reflects the provision for income taxes that would have
been recorded had MKS been a C corporation, assuming an effective tax rate of
39.0% for 1994 and 1995, and 38.0% for 1996, 1997, and 1998. As a result of
terminating its S corporation status upon the closing of this offering, MKS will
record a one-time non-cash credit to historical earnings for additional deferred
taxes. If this credit to earnings had occurred at December 31, 1998, the amount
would have been approximately $3.9 million. This amount is expected to change
through the closing of this offering and is excluded from pro forma net income.
See Notes 2 and 9 of Notes to Consolidated Financial Statements. Pro forma
balance sheet data reflects the liability for the distribution of an estimated
$35.9 million, calculated as of December 31, 1998, of cumulative undistributed S
corporation taxable income for which stockholders of record on the day prior to
effective date of the registration statement have been or will be taxed. The
actual amount to be distributed after the closing of this offering will be $40.0
million, the estimated amount of our cumulative undistributed S corporation
taxable income as of the day prior to the closing of this offering, subject to
adjustment. Pro forma net income per share for 1998 reflects the effect of an
assumed issuance of sufficient shares to fund the distribution, as of January 1,
1998. See "S Corporation and Termination of S Corporation Status" and Note 2 of
Notes to Consolidated Financial Statements.
    
 
<TABLE>
<CAPTION>
                                                      YEAR ENDED DECEMBER 31,
                                      --------------------------------------------------------
                                        1994        1995        1996        1997        1998
                                      --------    --------    --------    --------    --------
                                                           (IN THOUSANDS)
<S>                                   <C>         <C>         <C>         <C>         <C>
STATEMENT OF INCOME DATA:
Net sales...........................  $106,829    $157,164    $170,862    $188,080    $139,763
Cost of sales.......................    59,813      87,703     102,008     107,606      83,784
                                      --------    --------    --------    --------    --------
Gross profit........................    47,016      69,461      68,854      80,474      55,979
Research and development............     8,036      10,935      14,195      14,673      12,137
Selling, general and
  administrative....................    26,893      34,420      37,191      41,838      34,707
Restructuring.......................        --          --       1,400          --          --
                                      --------    --------    --------    --------    --------
Income from operations..............    12,087      24,106      16,068      23,963       9,135
Interest expense, net...............     1,284       1,448       2,286       1,861       1,187
Other income (expense), net.........        --          --        (479)        166         187
                                      --------    --------    --------    --------    --------
Income before income taxes..........    10,803      22,658      13,303      22,268       8,135
Provision for income taxes..........       800       1,000         800       1,978         949
                                      --------    --------    --------    --------    --------
Net income..........................  $ 10,003    $ 21,658    $ 12,503    $ 20,290    $  7,186
                                      ========    ========    ========    ========    ========
</TABLE>
 
                                       17
<PAGE>   19
 
   
     THE HISTORICAL NET INCOME PER SHARE DATA SET FORTH BELOW DOES NOT INCLUDE
PROVISIONS FOR FEDERAL INCOME TAXES BECAUSE PRIOR TO THE CLOSING OF THIS
OFFERING, MKS WAS TREATED AS AN S CORPORATION FOR FEDERAL INCOME TAX PURPOSES.
THE PRO FORMA STATEMENT OF INCOME DATA SET FORTH BELOW PRESENTS NET INCOME PER
SHARE DATA AS IF MKS HAD BEEN SUBJECT TO FEDERAL INCOME TAXES AS A C CORPORATION
DURING THE PERIODS PRESENTED.
    
 
<TABLE>
<CAPTION>
                                                            YEAR ENDED DECEMBER 31,
                                                     -------------------------------------
                                                     1994    1995    1996    1997    1998
                                                     -----   -----   -----   -----   -----
<S>                                                  <C>     <C>     <C>     <C>     <C>
HISTORICAL NET INCOME PER SHARE:
     Basic.........................................  $0.55   $1.20   $0.69   $1.12   $0.40
                                                     =====   =====   =====   =====   =====
     Diluted.......................................  $0.55   $1.20   $0.69   $1.10   $0.38
                                                     =====   =====   =====   =====   =====
</TABLE>
 
   
<TABLE>
<CAPTION>
                                                         YEAR ENDED DECEMBER 31,
                                            --------------------------------------------------
                                             1994       1995       1996       1997       1998
                                            -------    -------    -------    -------    ------
                                                  (IN THOUSANDS, EXCEPT PER SHARE DATA)
<S>                                         <C>        <C>        <C>        <C>        <C>
PRO FORMA STATEMENT OF INCOME DATA
  (UNAUDITED):
     Historical income before income
       taxes..............................  $10,803    $22,658    $13,303    $22,268    $8,135
     Pro forma provision for income taxes
       assuming C corporation tax.........    4,213      8,837      5,055      8,462     3,091
                                            -------    -------    -------    -------    ------
     Pro forma net income.................  $ 6,590    $13,821    $ 8,248    $13,806    $5,044
                                            =======    =======    =======    =======    ======
PRO FORMA NET INCOME PER COMMON SHARE:
     Basic................................  $  0.37    $  0.77    $  0.46    $  0.76    $ 0.25
                                            =======    =======    =======    =======    ======
     Diluted..............................  $  0.37    $  0.77    $  0.46    $  0.76    $ 0.24
                                            =======    =======    =======    =======    ======
</TABLE>
    
 
<TABLE>
<CAPTION>
                                            DECEMBER 31,                     DECEMBER 31, 1998
                             ------------------------------------------    ---------------------
                              1994        1995       1996        1997       ACTUAL     PRO FORMA
                             -------    --------    -------    --------    --------    ---------
                                                       (IN THOUSANDS)
<S>                          <C>        <C>         <C>        <C>         <C>         <C>
BALANCE SHEET DATA:
Cash and cash
  equivalents..............  $ 4,059    $  3,650    $ 3,815    $  2,511    $ 11,188    $ 11,188
Working capital
  (deficit)................   25,078      32,202     22,404      30,321      31,493      (4,433)
Total assets...............   72,320     104,511     95,000     106,536      96,232      96,232
Short-term obligations.....    9,246      15,192     16,124      13,852      12,819      12,819
Long-term obligations, less
  current portion..........   14,948      20,462     18,899      15,624      13,786      13,786
Stockholders' equity.......   37,272      48,392     45,498      52,848      54,826      18,900
</TABLE>
 
                                       18
<PAGE>   20
 
                    MANAGEMENT'S DISCUSSION AND ANALYSIS OF
                 FINANCIAL CONDITION AND RESULTS OF OPERATIONS
 
     The following discussion contains forward-looking statements that involve
risks and uncertainties. MKS's actual results could differ materially from those
discussed in the forward-looking statements as a result of certain factors
including those set forth under "Risk Factors" and elsewhere in this prospectus.
The following discussion and analysis should be read in conjunction with
"Selected Consolidated Financial Data" and the Consolidated Financial Statements
and Notes thereto appearing elsewhere in this prospectus.
 
OVERVIEW
 
     MKS was founded in 1961. MKS develops, manufactures and supplies
instruments and components used to measure, control and analyze gases in
semiconductor manufacturing and similar industrial manufacturing processes.
During 1997 and 1998, MKS estimates that approximately 60% of its net sales were
to semiconductor capital equipment manufacturers and semiconductor device
manufacturers. MKS expects that sales to such customers will continue to account
for a substantial majority of its sales. MKS's customers include semiconductor
capital equipment manufacturers, semiconductor device manufacturers, industrial
manufacturing companies and university, government and industrial research
laboratories. In 1996, 1997, and 1998, sales to MKS's top five customers
accounted for approximately 26%, 32% and 24%, respectively, of MKS's net sales.
During 1998, Applied Materials, Inc. accounted for approximately 16% of MKS's
net sales. MKS typically enters into contracts with its semiconductor equipment
manufacturer customers that provide for quantity discounts. MKS recognizes
revenue, and accrues for anticipated returns and warranty costs, upon shipment.
 
     In the third quarter of 1996, as a result of the downturn in the
semiconductor industry, MKS recorded a restructuring charge of $1.4 million. The
charge was primarily related to a reduction of personnel and the closure of
certain facilities and included the cost of severance, lease commitments and the
write-off of leasehold improvements. During 1998, as a result of the downturn in
the semiconductor industry, MKS reduced its staffing levels by approximately 30%
from its year-end 1997 levels.
 
     A significant portion of MKS's sales are to operations in international
markets. International sales by MKS's foreign subsidiaries, located in Japan,
Korea, Europe, and Canada, were 27.3% and 32.4% of net sales for 1997 and 1998,
respectively. Sales by MKS's Japan subsidiary comprised 15.0% and 15.1% of net
sales in 1997 and 1998, respectively. MKS does not classify export sales made
directly by MKS as international sales. Such export sales have generally been
less than 10% of net sales. MKS currently uses, and plans to continue to use,
forward exchange contracts and local currency purchased options to reduce
currency exposure arising from foreign denominated sales associated with the
intercompany purchases of inventory. Gains and losses on derivative financial
instruments that qualify for hedge accounting are classified in cost of sales.
Gains and losses on derivative financial instruments that do not qualify for
hedge accounting are marked-to-market and recognized immediately in other
income. See Note 3 to Notes to Consolidated Financial Statements.
 
     MKS has been treated as an S corporation for federal income tax purposes
since July 1, 1987. MKS's S corporation status will terminate upon the closing
of this offering, at which time MKS will become subject to federal, and certain
state, income taxation as a C corporation. The pro forma net income reflects a
pro forma effective tax rate of 38.0% to reflect federal and state income taxes
which would have been payable for 1998 had MKS been taxed as a C corporation.
See "S Corporation and Termination of S Corporation Status."
 
                                       19
<PAGE>   21
 
RESULTS OF OPERATIONS
 
     The following table sets forth for the periods indicated the percentage of
total net sales of certain line items included in MKS's consolidated statement
of income data:
 
<TABLE>
<CAPTION>
                                                              YEAR ENDED DECEMBER 31,
                                                              -----------------------
                                                              1996     1997     1998
                                                              -----    -----    -----
<S>                                                           <C>      <C>      <C>
Net sales...................................................  100.0%   100.0%   100.0%
Cost of sales...............................................   59.7     57.2     59.9
                                                              -----    -----    -----
Gross profit................................................   40.3     42.8     40.1
Research and development....................................    8.3      7.8      8.7
Selling, general and administrative.........................   21.8     22.3     24.9
Restructuring...............................................    0.8       --       --
                                                              -----    -----    -----
Income from operations......................................    9.4     12.7      6.5
Interest expense, net.......................................    1.3      1.0      0.8
Other income (expense), net.................................   (0.3)     0.1      0.1
                                                              -----    -----    -----
Income before income taxes..................................    7.8     11.8      5.8
Provision for income taxes..................................    0.5      1.0      0.7
                                                              -----    -----    -----
Net income..................................................    7.3%    10.8%     5.1%
                                                              =====    =====    =====
Pro forma data:
  Historical income before income taxes.....................    7.8%    11.8%     5.8%
  Pro forma provision for income taxes......................    3.0      4.5      2.2
                                                              -----    -----    -----
  Pro forma net income......................................    4.8%     7.3%     3.6%
                                                              =====    =====    =====
</TABLE>
 
Year Ended 1998 Compared to 1997
 
     Net Sales.  Net sales decreased 25.7% to $139.8 million for 1998 from
$188.1 million for 1997. International net sales were approximately $45.3
million in 1998 or 32.4% of net sales and $51.4 million in 1997 or 27.3% of net
sales. The decrease in net sales was primarily due to decreased sales volume of
MKS's existing products in the United States and in Asia caused by the 1998
downturn in the semiconductor capital equipment market.
 
     Gross Profit.  Gross profit as a percentage of net sales decreased to 40.1%
for 1998 from 42.8% in 1997. The change was primarily due to manufacturing
overhead costs being a higher percentage of net sales due to lower sales volume
in 1998.
 
     Research and Development.  Research and development expenses decreased
17.3% to $12.1 million or 8.7% of net sales for 1998 from $14.7 million or 7.8%
of net sales for 1997. The decrease was due to reduced spending for development
materials primarily related to certain projects that were completed during 1998.
 
     Selling, General and Administrative.  Selling, general and administrative
expenses decreased 17.0% to $34.7 million or 24.9% of net sales for 1998 from
$41.8 million or 22.3% of net sales for 1997. The decrease was due primarily to
a decrease of approximately $4.2 million in compensation expense resulting from
the reduction in personnel during 1998 and reduced incentive compensation.
Additionally, expenses were reduced as a result of lower spending on
advertising, travel, and other selling and administrative costs.
 
     Interest Expense, Net.  Net interest expense decreased to $1.2 million for
1998 from $1.9 million for 1997 primarily due to lower debt outstanding during
1998.
 
                                       20
<PAGE>   22
 
     Other Income (Expense), Net.  Other income of $0.2 million in 1998
primarily represents foreign exchange translation gains on intercompany payables
of $1.0 million offset by $0.7 million for costs associated with MKS's planned
initial public offering in early 1998 which was postponed. Other income of $0.2
million in 1997 represents gains of $1.2 million from foreign exchange contracts
that did not qualify for hedge accounting, offset by a foreign exchange
translation loss on an intercompany payable.
 
     Pro Forma Provision for Income Taxes.  The pro forma provision for income
taxes for 1998 reflects the estimated tax expense MKS would have incurred had it
been subject to federal and state income taxes as a C corporation under the
Internal Revenue Code. The pro forma provision reflects a pro forma tax rate of
38.0%, which differs from the federal statutory rate due primarily to the
effects of state and foreign taxes and certain tax credits.
 
Year Ended 1997 Compared to 1996
 
     Net Sales.  Net sales increased 10.1% to $188.1 million for 1997 from
$170.9 million for 1996. International net sales were approximately $51.4
million in both 1997 and 1996 and were 27.3% of net sales in 1997 and 30.1% of
net sales in 1996. The increase in net sales was primarily due to increased
sales volume of MKS's existing products in the United States.
 
     Gross Profit.  Gross profit as a percentage of net sales increased to 42.8%
for 1997 from 40.3% for 1996. The change was due primarily to the reduction in
fixed costs resulting from the restructuring effected in the third quarter of
1996 and the resulting increase in operational efficiencies.
 
     Research and Development.  Research and development expenses increased 3.4%
to $14.7 million or 7.8% of net sales for 1997 from $14.2 million or 8.3% of net
sales for 1996. The increase was primarily due to an increase in staffing
throughout 1997 for certain development projects.
 
     Selling, General and Administrative.  Selling, general and administrative
expenses increased 12.5% to $41.8 million or 22.3% of net sales for 1997 from
$37.2 million or 21.8% of net sales for 1996. The increase was due to increased
compensation expense resulting from increased salaries and wages and incentive
compensation.
 
     Restructuring.  In the third quarter of 1996, as a result of the downturn
in the semiconductor industry, MKS recorded a restructuring charge of $1.4
million. The charge included $0.4 million of severance pay, $0.7 million of
lease commitments, and $0.3 million for the write-off of leasehold improvements.
 
     Interest Expense, Net.  Net interest expense decreased to $1.9 million for
1997 from $2.3 million for 1996 primarily due to lower debt outstanding during
1997.
 
     Other Income (Expense), Net.  Other expense for 1996 and other income for
1997 reflect losses and gains of $0.5 million and $1.2 million, respectively,
from foreign exchange contracts that did not qualify for hedge accounting, and a
foreign exchange translation loss on an intercompany payable from MKS's Korean
subsidiary of $1.0 million related to the devaluation of the Korean won in the
fourth quarter of 1997.
 
     Pro Forma Provision for Income Taxes.  The pro forma provision for income
taxes for 1997 reflects the estimated tax expense MKS would have incurred had it
been subject to federal and state income taxes as a C corporation under the
Internal Revenue Code. The pro forma provision reflects a pro forma tax rate of
38.0%, which differs from the federal statutory rate due primarily to the
effects of state and foreign taxes and certain tax credits.
 
                                       21
<PAGE>   23
 
Selected Quarterly Operating Results
 
     The following tables present unaudited consolidated financial information
for the eight quarters ended December 31, 1998. In the opinion of management,
this information has been presented on the same basis as the audited
Consolidated Financial Statements appearing elsewhere in this prospectus. All
adjustments which management considers necessary for a fair presentation of the
results of such periods have been included to present fairly the unaudited
quarterly results when read in conjunction with MKS's Consolidated Financial
Statements and Notes thereto. The results for any quarter are not necessarily
indicative of future quarterly results of operations.
 
<TABLE>
<CAPTION>
                                                                         QUARTER ENDED
                                   -----------------------------------------------------------------------------------------
                                   MARCH 31,   JUNE 30,   SEPT. 30,   DEC. 31,   MARCH 31,   JUNE 30,   SEPT. 30,   DEC. 31,
                                     1997        1997       1997        1997       1998        1998       1998        1998
                                   ---------   --------   ---------   --------   ---------   --------   ---------   --------
                                                                        (IN THOUSANDS)
<S>                                <C>         <C>        <C>         <C>        <C>         <C>        <C>         <C>
STATEMENT OF INCOME DATA:
Net sales........................   $40,520    $45,749     $48,360    $53,451     $46,163    $34,026     $28,834    $30,740
Cost of sales....................    24,277     26,413      27,766     29,150      26,757     20,265      18,140     18,622
                                    -------    -------     -------    -------     -------    -------     -------    -------
Gross profit.....................    16,243     19,336      20,594     24,301      19,406     13,761      10,694     12,118
Research and development.........     2,994      3,563       3,779      4,337       3,794      3,107       2,568      2,668
Selling, general and
  administrative.................     9,612     10,321      10,816     11,089      10,112      9,045       7,808      7,742
                                    -------    -------     -------    -------     -------    -------     -------    -------
Income from operations...........     3,637      5,452       5,999      8,875       5,500      1,609         318      1,708
Interest expense, net............       494        527         445        395         375        337         234        241
Other income (expense), net......       275       (447)        632       (294)       (281)       123          77        268
                                    -------    -------     -------    -------     -------    -------     -------    -------
Income before income taxes.......     3,418      4,478       6,186      8,186       4,844      1,395         161      1,735
Provision for income taxes.......       289        378         523        788         565        163          19        202
                                    -------    -------     -------    -------     -------    -------     -------    -------
Net income.......................   $ 3,129    $ 4,100     $ 5,663    $ 7,398     $ 4,279    $ 1,232     $   142    $ 1,533
                                    =======    =======     =======    =======     =======    =======     =======    =======
</TABLE>
 
<TABLE>
<CAPTION>
                                                                         QUARTER ENDED
                                   -----------------------------------------------------------------------------------------
                                   MARCH 31,   JUNE 30,   SEPT. 30,   DEC. 31,   MARCH 31,   JUNE 30,   SEPT. 30,   DEC. 31,
                                     1997        1997       1997        1997       1998        1998       1998        1998
                                   ---------   --------   ---------   --------   ---------   --------   ---------   --------
<S>                                <C>         <C>        <C>         <C>        <C>         <C>        <C>         <C>
PERCENTAGE OF NET SALES:
Net sales........................     100.0%     100.0%      100.0%     100.0%      100.0%     100.0%      100.0%     100.0%
Cost of sales....................      59.9       57.7        57.4       54.5        58.0       59.6        62.9       60.6
                                    -------    -------     -------    -------     -------    -------     -------    -------
Gross profit.....................      40.1       42.3        42.6       45.5        42.0       40.4        37.1       39.4
Research and development.........       7.4        7.8         7.8        8.1         8.2        9.1         8.9        8.6
Selling, general and
  administrative.................      23.7       22.6        22.4       20.8        21.9       26.6        27.1       25.2
                                    -------    -------     -------    -------     -------    -------     -------    -------
Income from operations...........       9.0       11.9        12.4       16.6        11.9        4.7         1.1        5.6
Interest expense, net............       1.2        1.1         0.9        0.7         0.8        1.0         0.8        0.8
Other income (expense), net......       0.6       (1.0)        1.3       (0.6)       (0.6)       0.4         0.3        0.8
                                    -------    -------     -------    -------     -------    -------     -------    -------
Income before income taxes.......       8.4        9.8        12.8       15.3        10.5        4.1         0.6        5.6
Provision for income taxes.......       0.7        0.8         1.1        1.5         1.2        0.5         0.1        0.6
                                    -------    -------     -------    -------     -------    -------     -------    -------
Net income.......................       7.7%       9.0%       11.7%      13.8%        9.3%       3.6%        0.5%       5.0%
                                    =======    =======     =======    =======     =======    =======     =======    =======
</TABLE>
 
     MKS's quarterly operating results have varied significantly and are likely
to continue to vary significantly due to a number of factors including:
 
     - specific economic conditions in the industries in which MKS's customers
       operate, particularly the semiconductor industry
 
     - the timing of the receipt of orders from major customers
 
     - customer cancellations or shipment delays
 
     - price competition
 
     - disruption in sources of supply
 
     - seasonal variations of capital spending by customers
 
     - production capacity constraints
 
     - specific features requested by customers
 
                                       22
<PAGE>   24
 
     - exchange rate fluctuations
 
     - the introduction or announcement of new products by MKS or its
competitors
 
     - other factors, many of which are beyond MKS's control
 
     MKS's net sales have fluctuated over the past eight quarters primarily due
to the decline in the semiconductor capital equipment market and the
semiconductor device market in 1998 that adversely affected sales of MKS's
products in each of the quarters of 1998. MKS expects that the decline in
worldwide semiconductor capital equipment orders in the second half of 1998 and
the instability of the Asian markets will continue to adversely affect sales of
semiconductor capital equipment manufacturers for at least the first quarter of
1999. As a result, for at least the first quarter we currently expect that our
1999 quarterly net sales and net income will be less than net sales and net
income for the comparable quarter of 1998.
 
     Gross profit as a percentage of net sales increased in each quarter of 1997
primarily as a result of fuller utilization of existing manufacturing capacity
as a result of increased net sales. Gross profit as a percentage of net sales
decreased in each of the first three quarters of 1998 as a result of
manufacturing overhead costs becoming a higher percentage of net sales due to
lower sales volume.
 
     The increase in research and development expenses for the second, third and
fourth quarters of 1997 was primarily due to increased staffing levels. The
decrease in research and development expenses for the first, second, and third
quarters of 1998 was due to reduced spending for development materials primarily
related to certain projects that were completed during 1998.
 
     Selling, general and administrative expenses increased in the second, third
and fourth quarters of 1997 primarily due to increased compensation expense and
the write-off of certain abandoned assets. The decrease in selling, general and
administrative expenses in the first, second, and third quarters of 1998 was
primarily due to a decrease in compensation expense along with other selling
related expenses.
 
     Other income primarily represents gains and losses on foreign exchange
contracts and a foreign exchange translation loss on an intercompany payable
from MKS's Korean subsidiary of $1.0 million in the fourth quarter of 1997
related to the devaluation of the Korean won. Other expenses in the first
quarter of 1998 include $0.7 million for costs associated with MKS's planned
initial public offering in early 1998 which was postponed.
 
LIQUIDITY AND CAPITAL RESOURCES
 
     MKS has financed its operations and capital requirements through a
combination of cash provided by operations, long-term real estate financing,
capital lease financing and short-term lines of credit.
 
     Operations provided cash of $26.3 million, $16.8 million and $23.0 million
for 1996, 1997 and 1998, respectively, primarily impacted in each period by net
income, depreciation and changes in the levels of inventory and accounts
receivable. Investing activities utilized cash of $10.2 million, $3.3 million
and $2.1 million in 1996, 1997 and 1998, respectively, primarily for the
purchase of property and equipment in each period. Financing activities utilized
cash of $15.6 million, $16.2 million and $11.8 million in 1996, 1997 and 1998,
respectively, primarily for stockholder distributions in each period. Cash flows
from financing activities for each period were primarily from short-term and
long-term borrowings.
 
     Working capital was $31.5 million as of December 31, 1998. MKS has a
combined $30.0 million line of credit with two banks, expiring December 31,
1999, all of which is available. Interest on future borrowings under the line of
credit would be payable monthly at a rate based on LIBOR, which was 7.131% at
December 31, 1998. MKS also has lines of credit through its foreign subsidiaries
with several financial institutions totaling $15.0 million at December 31, 1998.
The total unused balance under these lines of credit was $5.3 million at
December 31, 1998. The interest rates on borrowings outstanding as of December
31, 1998 on these lines of credit ranged from 1.3% to 1.7%. Interest on future
borrowings under the unused balance of these lines of credit would be at rates
ranging from 1.5% to 7.85%. These lines generally expire and are renewed at six
month intervals. In addition, MKS has outstanding term loans and
 
                                       23
<PAGE>   25
 
mortgage loans from banks totaling $12.0 million (net of the current portion) at
December 31, 1998. See Notes 6 and 13 of Notes to Consolidated Financial
Statements.
 
   
     In 1997 and 1998, MKS distributed $12.4 million and $6.2 million,
respectively, of undistributed S corporation earnings to its stockholders. As
soon as practicable following the closing of this offering, MKS intends to make
a distribution to the stockholders of record on the day prior to the effective
date of the registration statement in the amount of $40.0 million, which is the
estimated balance of the accumulated adjustments account as of the day prior to
the closing of the offering, subject to adjustment. The accumulated adjustments
account is cumulatively equal to financial reporting income, adjusted for
differences between the methods of accounting used for financial accounting and
for federal income tax purposes from July 1, 1987 through the date of
termination of MKS's S corporation status, that has not been previously
distributed. Investors purchasing shares in this offering will not receive any
portion of the distribution. See "S Corporation and Termination of S Corporation
Status."
    
 
     MKS believes that the net proceeds from this offering, together with the
cash anticipated to be generated from operations and funds available from
existing credit facilities, will be sufficient to satisfy its estimated working
capital and planned capital expenditure requirements through at least the next
24 months.
 
EFFECT OF CURRENCY EXCHANGE RATES AND EXCHANGE RATE RISK MANAGEMENT
 
     A significant portion of MKS's business is conducted outside of the United
States through its foreign subsidiaries. The foreign subsidiaries maintain their
accounting records in their local currencies. Consequently, period to period
comparability of results of operations is affected by fluctuations in exchange
rates. MKS derives a significant portion of its cash flows from foreign
denominated revenue. To the extent the dollar value of foreign denominated
revenue is diminished as a result of a strengthening U.S. dollar, MKS's results
of operations and cash flows could be adversely affected.
 
     The primary currencies to which MKS has exposure are the Japanese yen and
the German mark. The nature of this exposure is from MKS selling inventory to
its overseas subsidiaries for resale in local currency. Consequently, the cash
flows from the overseas subsidiaries are affected by exchange rate fluctuations.
To reduce the risks associated with foreign currency rate fluctuations, MKS has
entered into forward exchange contracts and local currency purchased options on
a continuing basis in amounts and timing consistent with the underlying currency
exposures.
 
     The factors MKS considers in determining whether forward exchange contracts
or purchased options qualify for hedge accounting include:
 
     - whether the notional amounts of the derivatives offset the underlying
       currency exposures in terms of timing and amounts
 
     - for forward exchange contracts, whether the underlying transactions being
       hedged are pursuant to firm commitments
 
     - for local currency purchased options, whether it is probable that the
       underlying hedging transaction will occur
 
     Gains on forward exchange contracts and local currency purchased options,
qualifying for hedge accounting, amounted to $2.5 million, $1.2 million and $0.3
million for the years ended December 31, 1996, 1997 and 1998, respectively, and
are classified in cost of sales. Losses of $0.5 million, gains of $1.2 million
and losses of $0.2 million on forward exchange contracts that did not qualify
for hedge accounting were recognized in earnings for 1996, 1997 and 1998,
respectively, and are classified in other income (expense), net. These amounts
are net of a foreign exchange translation loss of $1.0 million and a gain of
$1.0 million on intercompany payables from its subsidiaries in 1997 and 1998
respectively. Foreign exchange translation gains and losses from unhedged
intercompany balances were not material in 1996.
 
                                       24
<PAGE>   26
 
While MKS does not issue or hold derivative financial instruments for trading
purposes, there can be no assurance that any losses realized on such instruments
will be fully offset by gains on the underlying exposure. Prospectively, MKS
plans to continue to use forward exchange contracts and local currency purchased
options to seek to mitigate the impact of exchange rate fluctuations. See Notes
2 and 3 of Notes to Consolidated Financial Statements.
 
MARKET RISK AND SENSITIVITY ANALYSIS
 
Foreign Exchange Rate Risk
 
     The potential fair value loss for a hypothetical 10% adverse change in
forward currency exchange rates on MKS's forward exchange contracts at December
31, 1998 would be $949,000. The potential loss was estimated by calculating the
fair value of the forward exchange contracts at December 31, 1998 and comparing
that with those calculated using the hypothetical forward currency exchange
rates.
 
     The value of the local currency purchased options at December 31, 1998 was
immaterial. Any loss related to the local currency purchased options is limited
to the unamortized premium of $155,000 at December 31, 1998.
 
     At December 31, 1998, MKS had $9,687,000 related to short-term borrowings
denominated in Japanese yen. The carrying value of these short-term borrowings
approximates fair value due to their short period to maturity. Assuming a
hypothetical 10% adverse change in the Japanese yen to U.S. dollar year end
exchange rate, the fair value of these short-term borrowings would increase by
$1,077,000. The potential increase in fair value was estimated by calculating
the fair value of the short-term borrowings at December 31, 1998 and comparing
that with the fair value using the hypothetical year end exchange rate.
 
Interest Rate Risk
 
     MKS is exposed to fluctuations in interest rates in connection with its
variable rate term loans. In order to minimize the effect of changes in interest
rates on earnings, MKS entered into an interest rate swap that fixed the
interest rate on its variable rate term loans. Under the swap agreement, MKS
pays a fixed rate of 5.85% on the notional amount and receives LIBOR. At
December 31, 1998, the notional amount of the interest rate swap was equal to
the principal amount of the variable rate term loans. The potential increase in
the fair value of term loans when adjusting for the interest rate swap paying at
a fixed rate resulting from a hypothetical 10% decrease in interest rates was
not material.
 
RECENTLY ISSUED ACCOUNTING PRONOUNCEMENTS
 
     See Note 2 of Notes to Consolidated Financial Statements for a discussion
of the impact of recently issued accounting pronouncements.
 
YEAR 2000 COMPLIANCE
 
     The Year 2000 problem stems from the fact that many currently installed
computer systems include software and hardware products that are unable to
distinguish 21st century dates from those in the 20th century. As a result,
computer software and/or hardware used by many companies and governmental
agencies may need to be upgraded to comply with Year 2000 requirements or risk
system failure or miscalculations causing disruptions to normal business
activities.
 
State of Readiness
 
     MKS designed and began implementation of a multi-phase Year 2000 project
which consists of:
 
     - assessment of the corporate systems and operations including both
       information technology and non-information technology that could be
       affected by the Year 2000 problem
 
     - remediation of non-compliant systems and components
 
                                       25
<PAGE>   27
 
     - testing of systems and components following remediation
 
     MKS, under the guidance of its Information Technology Steering Committee,
has focused its Year 2000 review on four areas:
 
     - internal computer software and hardware
 
     - product compliance
 
     - facilities and manufacturing equipment
 
     - third-party compliance
 
     Internal Computer Software and Hardware.  MKS uses information technology
for its internal infrastructure, which consists of its main enterprise systems
which include the systems used, in part, for purchase orders, invoicing,
shipping and accounting, and individual workstations, including personal
computers, and its network systems.
 
     Because MKS's business and manufacturing systems, such as its main
enterprise systems, are essential to its business, financial condition and
results of operations, MKS began its assessment of these systems prior to its
other non-critical information technology systems. MKS began its assessment in
the fall of 1997, and in November 1997, MKS developed a remediation plan for all
identified noncompliant business and manufacturing systems. This remediation
plan was implemented in January 1998. By July 1998, MKS had installed new
systems or upgraded existing systems. Based upon post-implementation testing and
review, management believes that all business and manufacturing systems within
its manufacturing operations are Year 2000 compliant.
 
     One of MKS's international subsidiaries is currently undergoing conversion
of its business systems in order to become Year 2000 compliant. Management
believes that these systems will be operational by June 1999. This phase of the
Year 2000 project is currently on schedule.
 
     MKS's personal computer based systems were assessed in early 1998. MKS
believes that all non-compliant hardware and software was identified by March
1998, at which time it made a list prioritizing databases to be remedied.
Critical databases were identified and were scheduled for remediation prior to
other databases. Remediation plans to convert the databases were initiated in
November 1998. MKS anticipates that it will complete its critical and
non-critical conversions by June 1999. This phase of the Year 2000 project is
currently on schedule.
 
     Product Compliance.  Throughout 1998, MKS assessed and addressed the Year
2000 compliance of its products. This assessment resulted in the identification
of MKS's products that were compliant and non-compliant. The substantial
majority of MKS's products were deemed to be compliant.
 
     The date related functions of all non-compliant products, other than
certain residual gas analysis products, are believed by MKS to be non-critical
in that such noncompliance would not affect the independent performance of the
product; would not cause the MKS product to cease operating on any particular
date; and independently would not pose a safety risk. MKS believes that Year
2000 problems associated with non-compliant residual gas analysis products will
also be non-critical. However, these products contain components of other
manufacturers and cannot be tested and therefore it is possible that such
products could cause unanticipated performance problems. The non-compliant
features of our other products primarily relate to non-essential functions such
as date displays. MKS made available to its customers a list which describes
Year 2000 readiness of its products. This phase of the Year 2000 project is
currently on schedule.
 
     Facilities and Manufacturing Equipment.  Some aspects of MKS's facilities
and manufacturing equipment may include embedded technology, such as
microcontrollers. The Year 2000 problem could cause a system failure or
miscalculation in such facilities or manufacturing equipment which could disrupt
MKS's operations. Affected areas include security systems, elevator controls,
voice mail and phone systems, clean room environmental controls, numerically
controlled production machinery and computer based production equipment. MKS
organized a team of experienced managers in November 1998 to assess the
potential problems in these areas. An assessment of all facilities and
manufacturing equipment was
 
                                       26
<PAGE>   28
 
conducted through December 1998, and a remediation plan was developed in January
1999. MKS anticipates completion of all corrective actions by June 1999 with
testing and review of corrected items to occur in the summer of 1999. This phase
of the Year 2000 project is currently on schedule.
 
     Third-Party Compliance.  MKS has relationships with third-parties including
customers and vendors and suppliers of goods, services and computer interfaces.
The failure of such persons to implement and execute Year 2000 compliance
measures in a timely manner, if at all, could, among other things:
 
     - adversely affect MKS's ability to obtain components in a timely manner
 
     - cause a reduction in the quality of components obtained by MKS
 
     - cause a reduction, delay or cancellation of customer orders received by
       MKS or a delay in payments by its customers for products shipped
 
     - result in the loss of services that would be necessary for MKS to operate
       in the normal course of business
 
     MKS assessed which of these third-party goods, services and interfaces were
critical to its operations and developed and mailed a standard survey to each
third-party deemed critical in January 1998. By March 1998, MKS had reviewed
most responses received. To date, the responses received indicate that the
third-parties are either in the process of developing remediation plans, or are
compliant. MKS anticipates further assessment to continue through March 1999 and
plans to conduct reviews at that time. A remediation plan is expected to be in
place by June 1999 with all critical third-parties achieving satisfactory
compliance by August 1999. This phase of the Year 2000 project is currently on
schedule.
 
Costs
 
     MKS's costs to date associated with assessment, remediation and testing
activities concerning the Year 2000 problem have been approximately $1,500,000.
MKS estimates that an additional $1,500,000, the major portion of which will be
capitalized and expensed over the life of the assets, will be required to
complete the replacement or modification of its facilities, manufacturing
equipment, computer software and products and to address the noncompliance of
key third-parties. MKS has funded and will continue to fund these activities
principally through cash provided by operations and existing leasing lines of
credit. It is not possible for MKS to completely estimate the costs incurred in
its remediation effort as many of its employees have focused and will continue
to focus significant efforts in evaluating MKS's Year 2000 state of readiness
and in remediating problems that have arisen, and will continue to arise, from
such evaluation.
 
Contingency Plan
 
     To date, MKS has not formulated contingency plans related to the failure of
its or a third-party's Year 2000 remediation efforts. Contingency plans for the
failure to implement compliance procedures have not been completed because it is
the intent of MKS to complete all required modifications and to test
modifications thoroughly prior to December 31, 1999. However, as discussed
above, MKS is engaged in ongoing assessment, remediation and testing activities
and the internal results as well as the responses received from third-parties
will be taken into account in determining the nature and extent of any
contingency plans if necessary.
 
                                       27
<PAGE>   29
 
                                    BUSINESS
 
     MKS is a leading worldwide developer, manufacturer and supplier of
instruments and components used to measure, control and analyze gases in
semiconductor manufacturing and similar industrial manufacturing processes. MKS
offers a comprehensive line of products which are used to manufacture, among
other things:
 
- - semiconductors
- - flat panel displays
- - magnetic and optical storage devices and media, including:
  -- compact disks
  -- hard disk storage devices
  -- magnetic devices for reading disk data
  -- digital video disks
  -- optical storage disks or laser readable disks
 
- - solar cells which convert light into electrical current
- - fiber optic cables for telecommunications
- - optical coatings, such as eyeglass coatings
- - coatings for architectural glass
- - hard coatings to minimize wear on cutting tools
- - diamond thin films
 
Our products include:
 
- - instruments used to measure, control and analyze:
  -- gas pressure
  -- gas flow
  -- gas composition
 
- - vacuum technology products:
  -- vacuum gauges
  -- vacuum valves and components
 
     For over 25 years, MKS has focused on satisfying the needs of semiconductor
capital equipment manufacturers and semiconductor device manufacturers and has
established long-term relationships with many of its customers. Over 4,000
customers worldwide purchased products from MKS during 1998 including:
 
     - semiconductor capital equipment manufacturers
 
     - semiconductor device manufacturers
 
     - industrial manufacturing companies
 
     - university, government and industrial research laboratories
 
     MKS's customers include Applied Materials, Inc., Lam Research Corporation,
Novellus Systems, Inc., Tokyo Electron Limited, Inc., Air Products and
Chemicals, Inc. and Motorola, Inc. MKS sells its products primarily through its
sales force which consists of 118 employees, as of December 31, 1998, in 22
offices in France, Germany, Japan, Korea, The Netherlands, Singapore, Taiwan,
the United Kingdom and the United States.
 
INDUSTRY BACKGROUND
 
     In the past 40 years, significant advances in materials science and
processing technologies have made possible the manufacture of products ranging
from highly complex microprocessor chips to simple but effective airtight
coatings for food packagings. In many materials processing applications,
specific gas mixtures at precisely controlled pressures are used:
 
     - to create and maintain the required process atmosphere
 
     - as a source of materials to be deposited on a surface, such as a silicon
       wafer
 
     - to remove or etch materials from a surface to form a circuit pattern
 
                                       28
<PAGE>   30
 
     The largest commercial application employing materials science and
processing technologies is the manufacture of semiconductors. Worldwide
semiconductor sales have increased as the use of semiconductors has expanded
beyond personal computers and computer systems to a wide array of additional
applications such as telecommunications and data communications systems,
automotive products, consumer goods, medical products and household appliances.
In large part, this growth has been facilitated by the ability of semiconductor
device manufacturers to produce increasingly fast, more complex, higher
performance semiconductors while steadily reducing cost per function, power
consumption requirements and size of these products to meet end-user and system
designer requirements. These improvements in the ratio of price to performance
have been enabled by advancements in semiconductor processing technologies,
which have facilitated the ability to reduce circuit pattern sizes and
subsequently increase the number of individual semiconductor circuits on a
silicon wafer. These trends have driven the need for increasingly complex and
sophisticated semiconductor device manufacturing processes, process equipment
and process controls.
 
  Semiconductor Manufacturing Process
 
     The manufacturing of semiconductors requires hundreds of process steps.
Many steps involve the controlled application or removal of layers of materials
to or from a surface referred to as a substrate. These process steps take place
within a process chamber, which provides a controlled environment for the
fabrication of semiconductor devices. Most of the key processes used in the
production of semiconductors require precise automatic control of gas pressure,
flow and composition in the process chamber.
 
     To ensure the integrity and performance of the manufacturing process,
semiconductor device manufacturers require sophisticated instruments that can
provide precise automated control of all major process variables within the
process chamber. The process steps required to produce circuit patterns involve
the control of multiple gases flowing into the process chamber at specified
intervals, and at controlled pressure and vacuum levels. In a typical process
step, the process chamber is evacuated to a base pressure established by a
vacuum pumping system and measured with vacuum gauges. Automatic shut-off valves
are sequenced to protect pumps and process instruments from exposure to
atmospheric pressure. Chamber leak integrity may be checked by gas analyzers
scanning for the presence of undesirable atmospheric gases or water vapor. Mass
flow controllers automatically control the flow rates of multiple gases into the
process chamber. Simultaneously, the automatic pressure control system for the
process chamber measures the pressure in the chamber and controls it at the
desired level by electronically adjusting the position of a control valve
located between the process chamber and the vacuum pump. Downstream of the
process chamber, heated lines, particle traps, and vacuum valves and switches
are used to prevent contamination of the process chamber as a result of the
backstream of particles and exhaust gases back into the process chamber. This
improves circuit quality, reduces maintenance and prolongs vacuum pump life.
 
                                       29
<PAGE>   31
 
     The pressures used in semiconductor manufacturing processes range from as
low as one trillionth of atmospheric pressure to as high as two hundred times
atmospheric pressure. The following table shows the wide range of pressures
required for typical semiconductor manufacturing processes:
    [PRESSURE RANGES OF TYPICAL SEMICONDUCTOR MANUFACTURING PROCESSES CHART]
[This table graphically depicts, using graybars, the gas pressure ranges, from
one trillionth of atmospheric pressure to two hundred times atmospheric pressure
used in various typical semiconductor manufacturing process steps (introduction
of gases into process chamber, deposition of materials and thin films on to
substrates, introduction of gases to etch circuit patterns, deposition of
conductive metal layers onto substrates and implantation of positively charged
atoms into substrates).
 
     The fabrication of a semiconductor circuit requires varying flow rates,
pressures and gases. A typical process step uses from three to five different
gases.
 
     Uptime, yield and throughput are critical semiconductor manufacturing
concepts. Uptime is the amount of time that the semiconductor processing tool is
available for processing. Yield is the ratio of acceptable circuits to total
circuits processed. Throughput is the number of wafers that can be processed per
hour. Uptime, yield, and throughput depend in major part upon:
 
     - precise repeatable measurement and control of the specific gas pressure,
       flow rates and composition
 
     - the maintenance of the vacuum integrity of the process chamber
 
     - the prevention of wafer contamination from particles entering the chamber
 
     Pressure variations of as little as one one-hundred-thousandth of
atmospheric pressure can change process yields significantly and errors in gas
flow rates and composition may impair circuit performance. Atmospheric
contamination and particle contamination can produce defects that significantly
reduce wafer yields and the time required to remove contaminates reduces uptime
and throughput. The speed of response and precision of the automatic control
systems directly affects uptime, throughput of wafers and process yields.
 
  Other Similar Industrial Manufacturing Processes
 
     Many of the same processes used to manufacture semiconductors are also used
to manufacture: flat panel displays; magnetic and optical storage devices and
media; solar cells; fiber optic cables for telecommunications; optical coatings;
coatings for architectural glass; hard coatings to minimize wear on cutting
tools; and diamond thin films.
 
                                       30
<PAGE>   32
 
  Trends in Semiconductor Manufacturing
 
     The ability of semiconductor device manufacturers to offer integrated
circuits with smaller geometries and greater functionality at higher speeds
requires continuous improvements in semiconductor process equipment and process
controls. The transition to smaller circuit patterns, such as 0.18 micron and
smaller line-widths, requires more process steps. It is also leading to the
introduction of new materials such as copper for conductors and a whole new
class of organic and inorganic materials for insulators. These in turn require
new technologies for delivery of gases and vapors to the process chamber. In
addition, the introduction of advanced processes such as high density plasma is
leading to a need for lower pressures, which are more difficult to measure and
control than higher pressures. These trends, along with increased wafer sizes,
which result in higher circuit value per wafer, are leading to the need for
increased sophistication of semiconductor processing equipment, a heightened
emphasis on uptime, yield and throughput and the need for more precise process
controls. As a result, the design and performance of instruments that control
pressure or the flow of gases, or analyze the composition of gases, are becoming
even more critical to the semiconductor manufacturing process.
 
     To address the increasing complexity of semiconductor devices,
semiconductor device manufacturers typically develop processes to create
particular device features using specific manufacturing equipment. The process
for each feature is then documented and may be subsequently replicated for use
in multiple fabrication facilities around the world. The precision,
repeatability and reliability of the measurement and control instrumentation
used for each process is critical to providing uptime, high yield and throughput
on manufacturing equipment at all facilities employing such processes.
Semiconductor device manufacturers are placing increasing importance on uptime,
yield, throughput and process consistency throughout their facilities to
minimize:
 
     - capital equipment expenditures
     - facility construction costs
     - overall ongoing operating costs
 
     The increasing sophistication of semiconductor devices requires an increase
in the number of components and subsystems used in the design of semiconductor
manufacturing process tools. To reduce manufacturing complexity, improve quality
and reliability and ensure long-term service and support, semiconductor capital
equipment manufacturers and semiconductor device manufacturers are increasingly
seeking to establish relationships with a smaller group of broad-based suppliers
that meet their needs on a worldwide basis and provide:
 
     - advanced technological capabilities to address the increasing
       complexities of the semiconductor manufacturing process
     - instrument and component designs that ensure repeatable processes around
       the world
     - value-added, integrated instruments and components
     - a worldwide sales, service and support infrastructure
 
MKS SOLUTION AND STRATEGY
 
     MKS's objective is to be the leading worldwide supplier of instruments and
components used to measure, control and analyze gases in semiconductor and other
advanced thin-film materials processing applications and to help semiconductor
device manufacturers achieve improvements in their return on invested capital.
The principal elements of MKS's solution and strategy to achieve this objective
are set forth below:
 
     Technology Leadership.  MKS's products incorporate leading-edge
technologies to control and monitor increasingly complex gas-related
semiconductor manufacturing processes, thereby enhancing
 
                                       31
<PAGE>   33
 
uptime, yield and throughput which can improve the investment return on capital
equipment and facilities. The instruments and components in MKS's product
offering provides the required capabilities through:
 
     - high precision operation over the extreme and variable pressure ranges
       required for semiconductor processes
 
     - precise, consistent and repeatable measurement and control performance
       that allows processes to be replicated in manufacturing facilities around
       the world
 
     - advanced control technologies which enhance uptime, yield and throughput
 
     - multiple, diverse and alternative technologies for controlling the flow
       rate and composition of gases and vapors needed for new classes of
       advanced materials for next generation semiconductor devices
 
     - innovative vacuum technology subsystems that reduce atmospheric and
       particle contamination, thereby enhancing uptime, yield and throughput
 
     MKS's products have continuously advanced as its customers' needs have
evolved. MKS seeks to extend its technological leadership by applying its
expertise in vacuum, pressure, flow and gas composition measurement control and
analysis technologies to develop advanced products that meet the critical gas-
related process requirements of semiconductor and advanced thin-film materials
manufacturers.
 
     MKS has introduced technological innovations including:
 
     - corrosion-resistant pressure and vacuum sensors
 
     - automatic pressure and vacuum control systems
 
     - compact single unit gas composition analyzers to replace bulky
       multi-component systems
 
     MKS has developed, and continues to develop, new products to address
emerging industry trends such as the transition from the use of 200mm wafers to
300mm wafers and the shrinking of integrated circuit line-widths from 0.25
micron to 0.18 micron and smaller. MKS has supplied pre-production equipment to
be incorporated into semiconductor capital equipment manufacturers' 300mm
pre-production semiconductor wafer process equipment, which is expected to be
included in pilot production lines of device manufacturers.
 
     MKS has also developed equipment that is being used by research
laboratories for semiconductor devices using less than 0.18 micron line-widths.
In addition, MKS has developed, and continues to develop, materials delivery
systems for new classes of materials, such as copper for conductors, titanium
nitride for barriers and a class of organic and inorganic dielectric materials
that are beginning to be used in small geometry manufacturing.
 
     MKS has been a leader in making its products compatible with emerging
digital network standards, such as DeviceNet. DeviceNet enables components used
in semiconductor manufacturing processes to transmit self-diagnostic and other
information on a digital host network. This reduces system complexity and space
requirements.
 
     To ensure that MKS maintains its leading-edge position, MKS aligns its
research and development program to the Semiconductor Industry Association
Technology Roadmap. The Semiconductor Industry Association Technology Roadmap
identifies technological developments, as well as obstacles, required to produce
future generations of semiconductor devices. MKS also maintains associations
with leading universities to anticipate future semiconductor production needs
three to seven years in advance.
 
     Comprehensive Product Offering.  MKS currently offers, and intends to
continue to offer, the widest range of pressure and vacuum measurement and
control products serving the semiconductor manufacturing and similar industrial
manufacturing industries. MKS offers a full line of products including a wide
range of gas pressure, flow and composition analysis measurement and control
instruments and vacuum gauges, valves and components.
 
     Since the development of its original Baratron laboratory-based pressure
measurement instrument in 1961, MKS has continuously enhanced and expanded its
product offerings in response to the evolving needs of its customers. For
example, MKS recently introduced the Micro Baratron instrument, a significantly
smaller version of its pressure measurement product, and a new low vapor
pressure material
 
                                       32
<PAGE>   34
 
delivery system. MKS plans to introduce new products throughout 1999, including
a line of mass flow calibrators and process monitoring hardware and software for
gas analysis.
 
     MKS's products are designed to meet the increasingly complex needs of its
customers. With the increasing sophistication of semiconductor capital equipment
leading to an increasing number of components and subsystems in semiconductor
manufacturing process tools, MKS delivers products that reduce equipment size
and improve process performance. MKS's subsystem products combine several
components into single integrated solutions. MKS's integrated solutions deliver
higher performance at a lower cost than similar subsystems built from discrete
components. Additionally, MKS's integrated solutions are easier to install and
configure, further reducing the overall cost to the customer.
 
     MKS plans to continue to expand its product lines through both internal
development and acquisitions of complementary businesses, products and
technologies. MKS's comprehensive product offering enables MKS to meet a broad
range of customer needs and provide a single source of solutions for
semiconductor device and semiconductor capital equipment manufacturers as they
seek to consolidate their supplier relationships to a smaller select group.
 
     Close Working Relationships with Customers.  MKS has focused on satisfying
the needs of semiconductor device manufacturers and semiconductor capital
equipment manufacturers for over 25 years and has established long-term
relationships with many of its customers. MKS works with its customers at the
pre-design and design stage to identify and respond to their requests for
current and future generations of products. These close working relationships
allow MKS to understand and address the cost and performance expectations of its
customers. MKS plans to enhance its relationships with its major customers and
identify opportunities to develop similar relationships with additional
semiconductor capital equipment manufacturers and semiconductor device
manufacturers.
 
     Applications in Related Markets.  MKS is leveraging its accumulated
expertise in the semiconductor industry by developing products for applications
that employ production processes similar to semiconductor fabrication processes
in their reliance upon gases and vacuum-based production technologies.
Applications served by MKS outside the semiconductor industry include vacuum
freeze-drying of pharmaceuticals and foods, sterilization of medical appliances,
and applications that involve advanced thin-film manufacturing such as flat
panel displays, magnetic and optical storage media, solar cells, fiber optic
cables and optical coatings. MKS plans to continue to identify and develop
products that address advanced materials processing applications where gas
management plays a critical role.
 
     Global Infrastructure and World Class Manufacturing Capabilities.  As
semiconductor device manufacturers have become increasingly global, they have
required that suppliers offer comprehensive local repair service and close
customer support. Manufacturers require close support to enable them to
calibrate, repair, modify, upgrade and retrofit their equipment to improve
process consistency, uptime, yield and throughput. To meet these market
requirements, MKS maintains a global sales and support organization with 22
offices worldwide. MKS currently manufactures its products at nine facilities in
the United States and abroad. MKS continues to devote significant resources to
expand and maintain its worldwide production and service capabilities to meet
the global demand for gas measurement, control and analysis instruments and
vacuum technology components. MKS opened a sales and support facility in
Singapore in 1998 and during 1999 plans to add manufacturing capabilities to its
Austin, Texas facility and further equip its cleanroom facilities in Andover and
Methuen, Massachusetts.
 
     MKS believes that the ability to manufacture reliable instruments and
components in a cost-effective manner is critical to meet the demanding
just-in-time delivery requirements of semiconductor capital equipment
manufacturers and semiconductor device manufacturers. MKS's worldwide production
and manufacturing facilities provide MKS with the ability to manufacture
reliable gas measurement, control and analysis instruments and components in a
timely and cost-effective manner. With a total of approximately 250,000 square
feet of manufacturing capacity in five locations in the United States and four
others in Germany, Japan, the United Kingdom and Korea, MKS has implemented
world class practices in quality and delivery techniques. MKS's manufacturing
facilities in the United States, the United Kingdom and Germany are ISO 9001
certified.
 
                                       33
<PAGE>   35
 
PRODUCTS
 
     MKS offers a full line of instruments and components that are used to
measure, control and analyze gases in semiconductor manufacturing and other
advanced thin-film manufacturing processes. MKS supplies products in two
principal areas:
 
     - measurement and control instrumentation products
 
     - vacuum technology products
 
     The following schematic shows where MKS products are used in a typical
semiconductor manufacturing process.
[CHART]
[Schematic showing where MKS products are used in a typical semiconductor
manufacturing process.]
 
     MEASUREMENT AND CONTROL INSTRUMENTATION PRODUCTS.  MKS designs and
manufactures a wide range of gas pressure, flow and composition analysis
measurement and control instrumentation. Each product line consists of products
which are designed for a variety of pressure, flow and composition ranges and
accuracies.
 
     Baratron Pressure Measurement Products.  MKS's Baratron pressure
measurement products are high precision, pressure measurement instruments. MKS
has five Baratron product families that range from high accuracy digital output
instruments to simple electronic switches. These products are typically used to
measure the pressure of the gases being distributed upstream of the process
chambers, to measure process chamber pressures and to measure pressures between
process chambers, vacuum pumps and exhaust lines. Baratron instruments measure
pressures at ranges from two hundred times atmospheric pressure to one billionth
of atmospheric pressure. MKS believes it offers the widest range of gas pressure
measurement instruments in the semiconductor and advanced thin-film materials
processing industries.
 
                                       34
<PAGE>   36
 
     A key feature of Baratron instruments is the ability to measure pressure
independent of gas composition, which is critical for precise pressure control
of semiconductor processes that involve gas mixtures. In these processes, there
is a need to control both pressure and gas mixture, but the pressure measurement
instrument must measure only the pressure of the sum of the gases in the
chamber, independent of gas composition. The Baratron instruments enable users
to achieve a highly precise, accurate and repeatable measurement of gas
pressure. Pressure measurement, independent of gas composition, is also useful
during process steps used to remove atmospheric gases as well as those used to
introduce specific amounts of various types of gases. Such processes are used to
manufacture fluorescent bulbs and to fabricate gas lasers.
 
     The following table shows MKS's principal Baratron pressure measurement
product lines:
 
                     BARATRON PRESSURE MEASUREMENT PRODUCTS
 
<TABLE>
<CAPTION>
- ----------------------------------------------------------------------------------------------------------
           PRODUCT LINES                              DESCRIPTION                    RANGES OF LIST PRICES
           -------------                              -----------                    ---------------------
<S>                                    <C>                                           <C>
High precision, high accuracy          Instruments with built-in temperature         $2,900-$6,400
pressure and vacuum measurement        stabilization features, for high
instruments                            precision, high accuracy and high
                                       temperature operation
- ----------------------------------------------------------------------------------------------------------
General purpose pressure and vacuum    Rugged instruments with and without           $450-$4,200
measurement instruments                built-in temperature stabilization
                                       features, for reliable, precise and
                                       accurate process measurement
- ----------------------------------------------------------------------------------------------------------
Ultra-clean high pressure and          Instruments with ultra- clean surfaces        $550-$1,050
vacuum measurement instruments         exposed to gas, for precise, high purity
                                       applications
- ----------------------------------------------------------------------------------------------------------
General purpose "MINI" pressure and    Small footprint instruments for precise,      $650-$1,400
vacuum measurement instruments         accurate, general purpose process
                                       measurement
- ----------------------------------------------------------------------------------------------------------
Electronic pressure and vacuum         Economical, stable instrument providing       $350-$750
switches                               "go/no-go" output for precise pressure
                                       trip-points and alarms
- ----------------------------------------------------------------------------------------------------------
</TABLE>
 
     MKS's list prices for its Baratron measurement products vary depending upon
precision, accuracy, pressure range, operating temperature range, stability and
gas purity specifications.
 
     Automatic Pressure and Vacuum Control Products.  MKS's automatic pressure
control products consist of analog and digital automatic pressure and vacuum
control electronic instruments and valves. These products enable precise control
of process pressure by electronically actuating valves which control the flow of
gases in and out of the process chamber to minimize the difference between
desired and actual pressure in the chamber. The electronic controllers vary from
simple analog units with precise manual tuning capability to state-of-the-art
self-tuning, digital signal processing controllers. The valve products vary from
small gas inlet valves to large exhaust valves.
 
                                       35
<PAGE>   37
 
     In most cases, MKS's Baratron pressure measurement instruments provide the
pressure input to the automatic pressure control device. Together, these
components create an integrated automatic pressure control system. MKS's
pressure control products can also accept inputs from other measurement
instruments, enabling the automatic control of gas input or exhaust based on
parameters other than pressure.
 
                 AUTOMATIC PRESSURE AND VACUUM CONTROL PRODUCTS
 
<TABLE>
<CAPTION>
- ----------------------------------------------------------------------------------------------------------
           PRODUCT LINES                              DESCRIPTION                    RANGES OF LIST PRICES
           -------------                              -----------                    ---------------------
<S>                                    <C>                                           <C>
Automatic throttle control valve       Analog controllers, self-tuning digital       $800-$2,650
controllers                            controllers and displayless self-tuning
                                       controllers
- ----------------------------------------------------------------------------------------------------------
Throttle control valves                Non-sealing and sealing valves; high speed    $1,400-$8,800
                                       sealing throttle control valves;
                                       automatic, microprocessor-based smart
                                       throttle control valves
- ----------------------------------------------------------------------------------------------------------
Automatic solenoid control valve       Stand-alone control electronics packages      $1,850-$2,900
controllers                            or integrated sensor, valve and control
                                       electronics packages
- ----------------------------------------------------------------------------------------------------------
Solenoid control valves                Elastomer and all-metal-sealed solenoid       $450-$1,500
                                       control valves
- ----------------------------------------------------------------------------------------------------------
</TABLE>
 
     MKS has recently introduced a line of integrated pressure controllers that
combine the functions of its Baratron pressure measurement instrument, flow
measurement instrument, control electronics and valve into a four-inch long
instrument which can be placed directly on a gas line to control pressure
downstream of the instrument while indicating the gas flow rate. This addresses
the need for smaller components, saving valuable clean room space.
 
     Flow Measurement and Control Products.  MKS's flow measurement products
include gas, vapor and liquid flow measurement products based upon thermal
conductivity, pressure and direct liquid injection technologies. The flow
control products combine the flow measuring device with valve control elements
based upon solenoid, piezo-electric and piston pump technologies. The products
measure and automatically control the mass flow rate of gases and vapors into
the process chamber. MKS's broad product lines include products that allow the
precise, automatic flow control of inert or corrosive gases, the automatic
control of low vapor pressure gases and heated liquid source materials, and the
automatic control of delicate, advanced technology liquid sources and vaporized
solid sources for next generation devices.
 
     MKS's line of thermal-based mass flow controllers, which control gas flow
based on the molecular weight of gases, includes all-metal-sealed designs and
ultra-clean designs for semiconductor applications, and general purpose
controllers for applications where all-metal-sealed construction is not
required. MKS has also developed pressure-based mass flow controllers, based on
Baratron pressure instrument measurement and control technology, which use flow
restrictors in the gas line to transform pressure control into mass flow
control.
 
                                       36
<PAGE>   38
 
                     FLOW MEASUREMENT AND CONTROL PRODUCTS
 
<TABLE>
<CAPTION>
- ----------------------------------------------------------------------------------------------------------
           PRODUCT LINES                              DESCRIPTION                    RANGES OF LIST PRICES
           -------------                              -----------                    ---------------------
<S>                                    <C>                                           <C>
Direct liquid injection subsystem      Pumps and vaporizes liquid precursors for     $8,500-$24,900
                                       metals and dielectrics into process
                                       chamber
- ----------------------------------------------------------------------------------------------------------
Gas box rate of rise calibrator        Measures pressure increase with time in a     $8,100-$11,800
                                       known volume
- ----------------------------------------------------------------------------------------------------------
Pressure-based vapor delivery          Measures and controls flow of low pressure    $4,900-$12,400
systems                                vapors into chamber
- ----------------------------------------------------------------------------------------------------------
Pressure-based mass flow               Gas flow controller consisting of Baratron    $2,700
controllers                            sensor, control valve, orifice and
                                       electronics
- ----------------------------------------------------------------------------------------------------------
Ultra-clean, all-metal-sealed          Gas flow controller consisting of sensor,     $1,400-$9,500
thermal mass flow controllers          control valve and electronics
- ----------------------------------------------------------------------------------------------------------
General purpose elastomer-sealed       Gas flow controller consisting of sensor,     $1,050-$2,450
mass flow controllers                  control valve and electronics
- ----------------------------------------------------------------------------------------------------------
</TABLE>
 
     Certain new materials required for the next generation of semiconductor
devices are difficult to control using traditional thermal mass flow technology.
To control these new materials, MKS has designed a direct liquid injection
subsystem which pumps a precise volume of liquid into a vaporizer, which in turn
supplies a controlled flow of vapor into the process chamber. The direct liquid
injection subsystem pump and vaporizer are presently used principally for
research and development applications for next generation semiconductor device
conductors, diffusion barriers and insulators, such as copper, titanium nitride
and dielectric materials.
 
     MKS's flow measurement products also include a calibration system which
independently measures mass flow and compares this measurement to that of the
process chamber mass flow controller. The demand for the MKS calibration system
is driven by the increasingly stringent process control needs of the
semiconductor industry and the need to reduce costly downtime resulting from
stopping operations to address mass flow controller problems.
 
     Gas Composition Analysis Instruments.  MKS's gas analysis instruments are
sold primarily to the semiconductor industry. The residual gas analysis product
lines include a quadrapole mass spectrometer sensor, which is a device that
separates gases based on molecular weight. MKS's quadrapole mass spectrometer
sensors include built-in electronics to analyze the composition of background
and process gases in the process chamber. MKS's ORION process monitoring system
is a sophisticated quadrapole mass spectrometer process analyzer for statistical
process monitoring of manufacturing processes operating from very low pressures
to atmospheric pressure. These instruments are provided both as portable
laboratory systems and as process gas monitoring systems used in the diagnosis
of semiconductor manufacturing process systems and are sold at prices ranging up
to $80,000. The gas monitoring systems can indicate out-of-bounds conditions,
such as the presence of undesirable atmospheric gases, water vapor or
out-of-tolerance amounts of specific gases in the process chamber, enabling
operators to diagnose and repair faulty equipment. MKS's gas sampling systems
provide a turn-key solution for withdrawing gases from chambers at relatively
high pressures for introduction into the low pressure gas analyzers. Next
generation semiconductor manufacturing processes, with smaller circuit patterns
and larger wafer sizes, are expected to require sophisticated gas analysis
instruments and/or monitoring equipment to ensure tighter process control and
earlier diagnosis of equipment malfunction.
 
                                       37
<PAGE>   39
 
     VACUUM TECHNOLOGY PRODUCTS.  MKS designs and manufactures a wide variety of
vacuum technology products, including vacuum gauges, vacuum valves and
components.
 
     Vacuum Gauging Products.  MKS offers a wide range of vacuum instruments
consisting of vacuum measurement sensors and associated power supply and readout
units. These vacuum gauges measure phenomena that are related to the level of
pressure in the process chamber and downstream of the process chamber between
the chamber and the pump. Unlike Baratron pressure measurement instruments,
vacuum gauges do not measure pressure directly. These gauges are used to measure
vacuum at pressures lower than those measurable with a Baratron pressure
measurement instrument or to measure vacuum in the Baratron pressure measurement
instrument range where less accuracy is required. MKS's indirect pressure gauges
use thermal conductivity and ionization gauge technologies to measure pressure
from atmospheric pressure to one trillionth of atmospheric pressure. MKS's
Baratron pressure measurement instruments, together with its vacuum gauges, are
capable of measuring the full range of pressures used in semiconductor and other
thin-film manufacturing processes from two hundred times atmospheric pressure to
one trillionth of atmospheric pressure.
 
     MKS also manufactures a wide range of vacuum gauge instruments in which the
associated electronics are packaged with the vacuum sensor, reducing panel space
and installation cost. MKS offers both analog and digital versions of these
vacuum gauge transducers.
 
     Vacuum Valves and Components.  MKS's vacuum valves are used on the gas
lines between the process chamber and the pump downstream of the process
chamber. MKS's vacuum components consist of flanges, fittings, traps and heated
lines that are used downstream from the process chamber to provide leak free
connections and to prevent condensable materials from depositing particles near
or back into the chamber. The manufacture of small circuit patterns cannot
tolerate contamination from atmospheric leaks or particles. MKS's vacuum
components are designed to minimize such contamination and thus increase yields
and uptimes.
 
                           VACUUM TECHNOLOGY PRODUCTS
 
<TABLE>
<CAPTION>
- ----------------------------------------------------------------------------------------------------------
           PRODUCT LINES                              DESCRIPTION                    RANGES OF LIST PRICES
           -------------                              -----------                    ---------------------
<S>                                    <C>                                           <C>
Cold cathode and hot filament          Electronic gauges to measure pressure down    $600-$6,200
vacuum gauges                          to one trillionth of atmospheric pressure
- ----------------------------------------------------------------------------------------------------------
Convection gauges                      Electronic gauges to measure from one         $200-$700
                                       atmosphere down to one millionth of
                                       atmospheric pressure
- ----------------------------------------------------------------------------------------------------------
Right-angle and in-line shut-off       High vacuum rapid action poppet valves        $250-$4,500
valves
- ----------------------------------------------------------------------------------------------------------
Vapor sublimation traps                Contaminant particle trap                     $1,800-$4,600
- ----------------------------------------------------------------------------------------------------------
Other vacuum components                Flanges, fittings, valves and heated lines    $50-$3,050
- ----------------------------------------------------------------------------------------------------------
</TABLE>
 
                                       38
<PAGE>   40
 
MARKETS AND APPLICATIONS
 
     MKS estimates that approximately 60% of its sales in 1998 were made to the
semiconductor industry. MKS's products are also used in other markets and
applications including the manufacture of, among other things:
 
     - flat panel displays
 
     - magnetic and optical storage devices and media
 
     - solar cells which convert light into electrical current
 
     - fiber optic cables for telecommunications
 
     - optical coatings, such as eyeglass coatings
 
     - coatings for architectural glass
 
     - hard coatings to minimize wear on cutting tools
 
     - diamond thin films
 
     MKS sells its products primarily through its direct sales force in 22
offices in France, Germany, Japan, Korea, The Netherlands, Singapore, Taiwan,
the United Kingdom and the United States. This direct sales force is
supplemented by sales representatives and agents in Canada, China, India,
Israel, and Italy and in selected U.S. cities. The major markets for MKS's
products include:
 
  Semiconductor Manufacturing
 
     MKS's products are sold to semiconductor capital equipment manufacturers
and semiconductor device manufacturers. MKS's products are used in the major
semiconductor processing steps such as:
 
     - depositing materials on to substrates
 
     - etching circuit patterns
 
     - implanting positively charged atoms into a substrate to alter electrical
       characteristics
 
     MKS's products are also used for process facility applications such as gas
distribution, pressure control and vacuum distribution in clean rooms where
semiconductor manufacturing takes place. MKS anticipates that the semiconductor
manufacturing market will continue to account for a substantial portion of its
sales. While the semiconductor device manufacturing market is global, the major
semiconductor capital equipment manufacturers are concentrated in the United
States, Japan and Europe.
 
  Flat Panel Display Manufacturing
 
     MKS's products are used in the manufacture of flat panel displays, which
require the same or similar fabrication processes as semiconductor
manufacturing. MKS sells its products both to flat panel original equipment
manufacturers and to end-users in the flat panel display market. The transition
to larger panel size and higher definition is driving the need for defect
reduction which requires tighter process controls. The major manufacturers for
flat panel displays and flat panel display equipment are concentrated in Japan.
 
                                       39
<PAGE>   41
 
  Magnetic and Optical Storage Devices and Media
 
     MKS's products are used in the manufacture of:
 
     - magnetic storage media which store and read data magnetically
 
     - optical storage media which store and read data using laser technology
 
     - compact disks
 
     - hard disks
 
     - data storage devices
 
     - digital video or versatile disks
 
     The transition to higher density storage capacity requires manufacturing
processes incorporating tighter process controls. While storage media
manufacturing is global, the major manufacturers are concentrated in Japan and
the Asia-Pacific region and storage media capital equipment manufacturers are
concentrated in the United States, Japan and Europe.
 
  Optical Fiber and Optical Coating
 
     MKS's products are used in optical fiber and optical thin-film coating
processes. MKS's products are sold both to coating equipment manufacturers and
to manufacturers of products made using optical thin-film coating processes.
Optical fibers used for data transmission are manufactured using processes to
deposit chemical vapors which are similar to those used in semiconductor
manufacturing. The requirement for greater data transmission is driving the need
for tighter control of optical fiber coating processes. Optical thin films for
eyeglasses, solar panels and architectural glass are deposited using processes
to deposit chemical vapors and gaseous metals similar to those used in
semiconductor manufacturing. Optical fiber manufacturing and optical thin-film
processing are concentrated in the United States, Japan and Europe.
 
  Other Coating Markets
 
     MKS's pressure and flow measurement and control instruments are also used
in processes for the application of thin films to harden tool bit surfaces, in
the production of diamond thin films, coatings for food container packagings and
coatings for jewelry and ornaments. The major equipment and process providers
are concentrated in the United States, Japan and Europe.
 
     MKS estimates that the flat panel display, magnetic and optical storage
media, optical fiber, optical coating markets and other coating markets
combined, accounted for approximately 12% and 14% of net sales for 1997 and
1998, respectively.
 
  Other Markets
 
     MKS's pressure measurement and control instruments and vacuum components
are used in plasma processes used to sterilize medical instruments, in vacuum
freeze drying of pharmaceuticals, foods and beverages, and in vacuum processes
involved in light bulb and gas laser manufacturing. MKS's products are also sold
to government, university and industrial laboratories for vacuum applications
involving research and development in materials science, physical chemistry and
electronics materials. The major equipment and process providers and research
laboratories are concentrated in the United States, Japan and Europe.
 
                                       40
<PAGE>   42
 
CUSTOMERS
 
     MKS's largest customers are leading semiconductor capital equipment
manufacturers such as Applied Materials, Lam Research, Novellus and Tokyo
Electron, semiconductor device manufacturers such as Motorola, and specialty gas
providers such as Air Products and Chemicals. In 1996, 1997, and 1998, sales to
MKS's top five customers accounted for approximately 26%, 32% and 24%,
respectively, of MKS's net sales. During the same periods, international sales
represented approximately 30%, 27% and 32% of total net sales, respectively.
During 1998, Applied Materials accounted for approximately 16% of MKS's net
sales. Applied Materials purchases products from MKS under the terms of an
agreement, with no minimum purchase requirements, that expires in 2000.
 
SALES, MARKETING AND SUPPORT
 
     MKS's worldwide sales, marketing and support organization is critical to
its strategy of maintaining close relationships with semiconductor capital
equipment manufacturers and semiconductor device manufacturers. MKS sells its
products primarily through its direct sales force. As of December 31, 1998, MKS
had 118 sales employees in 22 offices in France, Germany, Japan, Korea, The
Netherlands, Singapore, Taiwan, the United Kingdom and the United States. This
direct sales force is supplemented by sales representatives and agents in
Canada, China, India, Israel, and Italy and in selected U.S. cities. MKS
maintains a marketing staff, which as of December 31, 1998, consisted of 14
employees, to identify customer requirements, assist in product planning and
specifications and to focus on future trends in the semiconductor and other
markets.
 
     As semiconductor device manufacturers have become increasingly sensitive to
the significant costs of system downtime, they have required that suppliers
offer comprehensive local repair service and close customer support.
Manufacturers require close support to enable them to repair, modify, upgrade
and retrofit their equipment to improve yields and adapt new materials or
processes. To meet these market requirements, MKS maintains a worldwide sales
and support organization with offices in 22 locations. Technical support is
provided by applications engineers located at offices in Arizona, California,
Colorado, Massachusetts, Oregon and Texas, as well as Canada, France, Germany,
India, Israel, Italy, Japan, Korea, The Netherlands, Singapore, Taiwan and the
United Kingdom. Repair and calibration services are provided at 14 service
depots located worldwide. MKS provides warranties from one to three years,
depending upon the type of product. In addition, MKS offers training programs
for its customers in a wide range of vacuum and gas processing technologies.
 
MANUFACTURING
 
     MKS believes that the ability to manufacture reliable gas management
instruments and components in a cost-effective manner is critical to meeting the
demanding requirements of semiconductor capital equipment manufacturers and
semiconductor device manufacturers. MKS monitors and analyzes product lead
times, warranty data, process yields, supplier performance, field data on mean
time between failures, inventory turns, repair response time and other
indicators so that it may continuously improve its manufacturing processes. MKS
has adopted a total quality management process. MKS's manufacturing facilities
in the United States, the United Kingdom and Germany are ISO 9001 certified.
 
     MKS is devoting significant financial and management resources to maintain
and expand its worldwide production and service capabilities to meet the global
demand for gas management instruments and components. MKS believes that the
ability to manufacture reliable instruments and components in a cost-effective
manner is critical to meet the demanding just-in-time delivery requirements of
semiconductor capital equipment manufacturers and semiconductor device
manufacturers. Due to the short time between the receipt of orders and
shipments, MKS normally operates with a level of backlog that is not
significant. MKS currently manufactures its products at nine facilities in the
United States and abroad. MKS plans to add manufacturing capabilities in 1999 to
its Austin, Texas facilities and further equip its cleanroom facilities in
Andover and Methuen, Massachusetts.
 
     MKS's principal manufacturing activities consist of precision assembly,
test, calibration, welding and machining activities. MKS subcontracts a portion
of its assembly, machining and printed circuit board assembly and testing. All
other assembly, test and calibration functions are performed by MKS. Critical
assembly activities are performed in cleanroom environments at MKS's facilities.
 
                                       41
<PAGE>   43
 
RESEARCH AND DEVELOPMENT
 
     MKS's research and development efforts are directed toward developing and
improving MKS's gas management instruments and components for semiconductor and
advanced thin-film processing applications and identifying and developing
products for new applications for which gas management plays a critical role.
MKS has undertaken an initiative to involve its marketing, engineering,
manufacturing and sales personnel in the concurrent development of new products
in order to reduce the time to market for new products. MKS's employees also
work closely with its customers' development personnel. These relationships help
MKS identify and define future technical needs on which to focus its research
and development efforts. In addition, MKS participates in SEMI/SEMATECH, a
consortium of semiconductor equipment suppliers, to assist in product
development and standardization of product technology, and it supports research
at academic institutions targeted at advances in materials science and
semiconductor process development.
 
     As of December 31, 1998, MKS employed a research and development staff of
89 employees. In 1996, 1997 and 1998, MKS's research and development
expenditures were approximately $14.2 million, $14.7 million and $12.1 million,
respectively, representing approximately 8.3%, 7.8% and 8.7% of net sales,
respectively.
 
COMPETITION
 
     The market for MKS's products is highly competitive. Principal competitive
factors include:
 
     - historical customer relationships
 
     - product quality, performance and price
 
     - breadth of product line
 
     - manufacturing capabilities
 
     - customer service and support
 
     While MKS believes that it competes favorably with respect to these
factors, there can be no assurance that it will continue to do so.
 
     MKS encounters substantial competition in each of its product lines from a
number of competitors, although no one competitor competes with MKS across all
product lines. Certain of MKS's competitors have greater financial and other
resources than MKS. In some cases, the competitors are smaller than MKS, but
well-established in specific product niches. Millipore Corporation offers
products that compete with MKS's pressure and flow products. Aera Corporation,
STEC (Horiba Ltd.), and Unit Instruments, Inc., each offer products that compete
with MKS's mass flow control products. Nor-Cal Products, Inc. and MDC Vacuum
Products, Inc., each offer products that compete with MKS's vacuum components.
Leybold-Inficon, Inc., offers products that compete with MKS's vacuum measuring
and gas analysis products. Helix Technology Corporation offers products that
compete with MKS's vacuum gauging products. Spectra International LLC offers
products that compete with MKS's gas analysis products.
 
     In some cases, particularly with respect to mass flow controllers,
semiconductor device manufacturers may direct semiconductor capital equipment
manufacturers to use a specified supplier's product in their equipment.
Accordingly, MKS's success depends in part on its ability to have semiconductor
device manufacturers specify that its products be used at their fabrication
facilities and MKS may encounter difficulties in changing established
relationships of competitors with a large installed base of products at such
customers' fabrication facilities. In addition, MKS's competitors can be
expected to continue to improve the design and performance of their products.
There can be no assurance that competitors will not develop products that offer
price or performance features superior to those of MKS's products.
 
                                       42
<PAGE>   44
 
PATENTS AND OTHER INTELLECTUAL PROPERTY RIGHTS
 
     MKS relies on a combination of patent, copyright, trademark and trade
secret laws and license agreements to establish and protect its proprietary
rights. MKS has 49 U.S. patents and 8 pending U.S. patent applications. Foreign
counterparts of certain of these applications have been filed or may be filed at
the appropriate time. While MKS believes that certain patents may be important
for certain aspects of its business, MKS believes that its success depends more
upon close customer contact, innovation, technological expertise, responsiveness
and worldwide distribution.
 
     MKS requires each of its employees, including its executive officers, to
enter into standard agreements pursuant to which the employee agrees to keep
confidential all proprietary information of MKS and to assign to MKS all
inventions made while in the employ of MKS.
 
EMPLOYEES
 
     As of December 31, 1998, MKS employed 821 persons, including 486 in
manufacturing, 89 in research and development, 246 in marketing, sales, support
and general and administrative activities. Management believes that MKS's
ongoing success depends upon its continued ability to attract and retain highly
skilled employees. None of MKS's employees is represented by a labor union or
party to a collective bargaining agreement. MKS believes that its employee
relations are good.
 
FACILITIES
 
     MKS sells its products primarily through its direct sales force in 22
offices in France, Germany, Japan, Korea, The Netherlands, Singapore, Taiwan,
the United Kingdom and the United States. The direct sales force is supplemented
by sales representatives and agents in Canada, China, India, Israel, and Italy
and in selected U.S. cities. MKS's corporate headquarters are located in
Andover, Massachusetts. Manufacturing and other operations are conducted in a
number of locations worldwide. MKS's minimum payments for leased real estate for
the year ending December 31, 1999 are expected to be $1,484,000. MKS believes
that the current facilities along with the planned addition for 1999 will be
adequate and suitable to meet its needs for the foreseeable future. The
following table provides information concerning MKS's principal and certain
other owned and leased facilities:
 
<TABLE>
<CAPTION>
                                                                                       LEASE
       LOCATION          SQ. FT.          ACTIVITY           PRODUCTS MANUFACTURED    EXPIRES
       --------          -------          --------           ---------------------   ---------
<S>                      <C>       <C>                      <C>                      <C>
Andover, Massachusetts   82,000    Headquarters,            Baratron pressure           (1)
                                   Manufacturing, Customer  measurement products
                                   Support and Research &
                                   Development
Boulder, Colorado        86,000    Manufacturing, Customer  Vacuum gauges, valves       (2)
                                   Support, Service and     and components
                                   Research & Development
Methuen, Massachusetts   85,000    Manufacturing, Customer  Pressure control and        (1)
                                   Support, Service and     flow measurement and
                                   Research & Development   control products
Lawrence, Massachusetts  40,000    Manufacturing            Baratron pressure           (1)
                                                            measurement products
Tokyo, Japan             20,700    Manufacturing, Sales,    Mass flow measurement       (3)
                                   Customer Support,        and control products
                                   Service and Research &
                                   Development
Santa Clara, California  15,600    Sales, Customer Support  Not applicable             (4)*
                                   and Service
</TABLE>
 
                                       43
<PAGE>   45
 
<TABLE>
<CAPTION>
                                                                                       LEASE
       LOCATION          SQ. FT.          ACTIVITY           PRODUCTS MANUFACTURED    EXPIRES
       --------          -------          --------           ---------------------   ---------
<S>                      <C>       <C>                      <C>                      <C>
Richardson, Texas        14,600    Manufacturing, Sales,    Subassemblies             8/31/01
                                   Customer Support and
                                   Service
Munich, Germany          14,100    Manufacturing, Sales,    Mass flow measurement       (1)
                                   Customer Support,        and control products
                                   Service and Research &
                                   Development
Le Bourget, France       13,700    Sales, Customer Support  Not applicable              (1)
                                   and Service
Austin, Texas             8,200    Sales, Customer Support  Not applicable            1/30/03
                                   and Service
Seoul, Korea              4,760    Manufacturing, Sales,    Mass flow measurement    5/30/00**
                                   Customer Support and     and control products
                                   Service
Manchester, U.K.          2,200    Manufacturing, Sales,    Mass flow measurement     10/5/09
                                   Customer Support and     and control products
                                   Service
Singapore                 2,050    Sales, Customer Support  Not applicable            3/25/01
                                   and Service
Taiwan                    2,050    Sales, Customer Support  Not applicable           12/31/01
                                   and Service
</TABLE>
 
- ---------------
(1) This facility is owned by MKS.
 
(2) MKS leases one facility which has 39,000 square feet of space and a lease
    term which expires 10/31/01 and owns a second and third facility with 28,000
    and 19,000 square feet of space, respectively.
 
(3) MKS leases a facility which has 14,000 square feet of space and a lease term
    which expires 4/30/99 and owns another facility with 6,700 square feet of
    space.
 
(4) MKS leases one facility with 4,000 square feet of space on a month-to-month
    basis, a second facility of 4,000 square feet with a lease term which
    expires on 1/30/00 and a third facility of 2,600 square feet with a lease
    term which expires 6/30/99. MKS owns a fourth facility of 5,000 square feet.
 
 *  MKS has an option to extend its leases at this location for a period of 18
    months.
 
**  MKS has an option to extend this lease for a period of two years.
 
     In addition to manufacturing and other operations conducted at the
foregoing leased or owned facilities, MKS provides worldwide sales, customer
support and services from various other leased facilities throughout the world
not listed in the table above. See "Business -- Sales, Marketing and Support."
 
LEGAL PROCEEDINGS
 
     MKS is not a party to any material legal proceedings.
 
                                       44
<PAGE>   46
 
                                   MANAGEMENT
 
EXECUTIVE OFFICERS AND DIRECTORS
 
     The executive officers and directors of MKS as of December 31, 1998 are as
follows:
 
   
<TABLE>
<CAPTION>
NAME                                        AGE                         POSITION
- ----                                        ---                         --------
<S>                                         <C>   <C>
John R. Bertucci..........................  57    Chairman, Chief Executive Officer and President
Ronald C. Weigner.........................  53    Vice President and Chief Financial Officer
John J. Sullivan..........................  63    Executive Vice President of Technology
William D. Stewart........................  54    Corporate Vice President and General Manager, Vacuum
                                                  Products
Joseph A. Maher, Jr.......................  51    Corporate Vice President and General Manager,
                                                  Measurement and Control Products
Leo Berlinghieri..........................  45    Corporate Vice President, Customer Support
                                                  Operations
Richard S. Chute(1).......................  60    Director
Owen W. Robbins(2)........................  69    Director
Robert J. Therrien........................  64    Director
Louis P. Valente(1)(2)....................  68    Director
</TABLE>
    
 
- ---------------
(1) Member of Compensation Committee.
 
(2) Member of Audit Committee.
 
     Mr. Bertucci has served as President and a Director of MKS since 1974 and
has been Chairman of the Board of Directors and Chief Executive Officer since
November 1995. From 1970 to 1974, he was Vice President and General Manager. Mr.
Bertucci has an M.S. in Industrial Administration and a B.S. in Metallurgical
Engineering from Carnegie-Mellon University. Mr. Bertucci is also a director of
Applied Science and Technology Corporation and Intellisense Corporation.
 
     Mr. Weigner has served as Vice President and Chief Financial Officer of MKS
since November 1995. From September 1993 until November 1995, he was Vice
President and Corporate Controller and from 1980 to 1993 he was Corporate
Controller. Mr. Weigner is a certified public accountant and has a B.S. in
Business Administration from Boston University.
 
     Mr. Sullivan has served as Executive Vice President of Technology of MKS
since March 1995. From 1982 to March 1995, he was Vice President of Marketing,
and from 1975 to 1982, he was Vice President of Sales and Marketing. Mr.
Sullivan has an M.S. and a B.S. in Physics from Northeastern University.
 
     Mr. Stewart has served as Corporate Vice President of MKS and General
Manager of Vacuum Products since November 1997. From October 1986 to November
1997, he was President of HPS Vacuum Products group, which MKS acquired in
October 1986. Mr. Stewart co-founded HPS in 1976. Mr. Stewart has an M.B.A. from
Northwestern University and a B.S. in Business Administration from the
University of Colorado. Mr. Stewart also serves on the board of directors of the
Janus Fund.
 
     Mr. Maher has served as Corporate Vice President of MKS and General Manager
of Measurement and Control Products since November 1997. From March 1997 through
November 1997, he served as Vice President of the Process Control
Instrumentation Group. Mr. Maher was a Vice President of Lam Research
Corporation from 1993 through 1996, and from 1980 through 1993, he was Executive
Vice President of Drytek Corporation, which was purchased by Lam Research
Corporation in 1993. Mr. Maher has a B.S. in Electrical Engineering from
Northeastern University.
 
     Mr. Berlinghieri has served as Corporate Vice President, Customer Support
Operations of MKS since November 1995. From 1980 to November 1995, he served in
various management positions at MKS, including Manufacturing Manager, Production
& Inventory Control Manager, and Director of Customer
 
                                       45
<PAGE>   47
 
Support Operations. Mr. Berlinghieri is also Treasurer of the TQM-BASE Council,
Inc., a non-profit quality management consortium comprised of Boston-area
semiconductor capital equipment manufacturers.
 
     Mr. Chute has served as a director of MKS since 1974. Mr. Chute has been a
member of the law firm of Hill & Barlow, a professional corporation, since
November 1971.
 
     Mr. Robbins has served as a director of MKS since February 1996. Mr.
Robbins was Executive Vice President of Teradyne, Inc., a manufacturer of
electronic test systems and backplane connection systems used in the electronics
and telecommunications industries from March 1992 to May 1997, and its Chief
Financial Officer from February 1980 to May 1997. Mr. Robbins has served on the
board of directors of Teradyne, Inc. since March 1992 and was its Vice Chairman
from January 1996 to May 1997.
 
     Mr. Therrien has served as a director of MKS since February 1996. Mr.
Therrien has been President and Chief Executive Officer of Brooks Automation,
Inc., a manufacturer of semiconductor processing equipment, since 1989.
 
     Mr. Valente has served as a director of MKS since February 1996. Mr.
Valente has been Chairman and Chief Executive Officer of Palomar Medical
Technologies, Inc., a company which designs, manufactures and markets cosmetic
lasers, since September 1997. He has been a director of Palomar Medical
Technologies, Inc. since February 1997 and was its President and Chief Executive
Officer from May 1997 to September 1997. Mr. Valente was a Senior Vice President
of Acquisitions, Mergers and Investments of EG&G, Inc. from 1991 until July
1995. Mr. Valente is also a director of Micrion Corporation.
 
     Executive officers of MKS are elected by the Board of Directors on an
annual basis and serve until their successors are duly elected and qualified.
There are no family relationships among any of the executive officers of MKS.
 
COMMITTEES OF THE BOARD OF DIRECTORS
 
     The Compensation Committee consists of Messrs. Chute and Valente. The
Compensation Committee reviews and evaluates the salaries, supplemental
compensation and benefits of all officers of MKS, reviews general policy matters
relating to compensation and benefits of employees of MKS and makes
recommendations concerning these matters to the Board of Directors. The
Compensation Committee also administers MKS's stock option and stock purchase
plans. See "-- Stock Plans."
 
     The Audit Committee consists of Messrs. Robbins and Valente. The Audit
Committee reviews with MKS's independent auditor the scope and timing of its
audit services, the auditor's report on MKS's financial statements following
completion of its audit and MKS's policies and procedures with respect to
internal accounting and financial controls. In addition, the Audit Committee
will make annual recommendations to the Board of Directors for the appointment
of independent auditors for the ensuing year.
 
DIRECTOR COMPENSATION
 
     Directors of MKS are reimbursed for expenses incurred in connection with
their attendance at Board of Directors and committee meetings. Directors who are
not employees of MKS are paid an annual fee of $10,000 and $1,000 for each Board
of Directors meeting they attend and $500 for each committee meeting they attend
which is not held on the same day as a Board of Directors meeting. Messrs.
Chute, Robbins, Therrien and Valente, MKS's four non-employee directors, have
each been granted options, under MKS's 1996 Director Stock Option Plan (under
which no further grants will be made), to purchase 8,592 shares of common stock
at a weighted average exercise price of $4.81 per share. Each has also been
granted options to purchase 6,000 shares of common stock at an exercise price of
$14.40 per share under the 1997 Director Stock Option Plan.
 
                                       46
<PAGE>   48
 
COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION
 
     The Compensation Committee is currently comprised of Messrs. Chute and
Valente. No member of the Compensation Committee was at any time an employee of
MKS. No executive officer of MKS serves as a member of the Board of Directors or
Compensation Committee of any other entity which has one or more executive
officers serving as a member of MKS's Board of Directors or Compensation
Committee.
 
EXECUTIVE COMPENSATION
 
     The following table sets forth information with respect to the compensation
of MKS's Chief Executive Officer and each of the four other most highly
compensated executive officers for the year ended December 31, 1998 (the "Named
Executive Officers").
 
                      SUMMARY COMPENSATION TABLE FOR 1998
 
<TABLE>
<CAPTION>
                                                                      LONG-TERM
                                                                     COMPENSATION
                                                                        AWARDS
                                     ANNUAL COMPENSATION             ------------
                             ------------------------------------     SECURITIES
                                                     OTHER ANNUAL     UNDERLYING        ALL OTHER
NAME AND PRINCIPAL POSITION   SALARY      BONUS      COMPENSATION     OPTIONS(#)     COMPENSATION(1)
- ---------------------------  --------    --------    ------------    ------------    ---------------
<S>                          <C>         <C>         <C>             <C>             <C>
John R. Bertucci...........  $337,440          --            --              --          $12,264
  Chief Executive Officer
  and President
Ronald C. Weigner..........   164,257          --            --          60,000            8,000
  Vice President and Chief
  Financial Officer
Joseph A. Maher, Jr........   161,307          --            --          60,000            8,000
  Corporate Vice President
  and General Manager,
  Measurement and Control
  Products
William D. Stewart.........   173,893          --            --          60,000            8,000
  Corporate Vice President
  and General Manager,
  Vacuum Products
Leo Berlinghieri...........   152,559          --            --          60,000            3,200
  Corporate Vice President,
  Customer Support
  Operations
</TABLE>
 
- ---------------
(1) Includes a premium of $4,264 paid on a life insurance policy and estimated
    payments of $8,000 paid into a 401(k) plan for Mr. Bertucci, and estimated
    payments paid into a 401(k) plan for Messrs. Weigner, Maher, Stewart and
    Berlinghieri.
 
                                       47
<PAGE>   49
 
STOCK OPTION GRANTS
 
     The following table contains information concerning the grants of options
to purchase MKS's common stock made to each of the Named Executive Officers for
the year ended December 31, 1998. Stock options are generally granted at 100% of
the fair value of MKS's common stock as determined by the Board of Directors on
the date of grant. In reaching the determination of fair value at the time of
each grant, the Board of Directors considers a range of factors, including MKS's
current financial position, its recent revenues, results of operations and cash
flows, its assessment of MKS's competitive position in its markets and prospects
for the future, the status of MKS's product development and marketing efforts,
current valuations for comparable companies and the illiquidity of an investment
in MKS's common stock.
 
                             OPTION GRANTS IN 1998
 
<TABLE>
<CAPTION>
                                                  INDIVIDUAL GRANTS
                              ---------------------------------------------------------   POTENTIAL REALIZABLE VALUE AT
                               NUMBER OF                                                     ASSUMED ANNUAL RATES OF
                              SECURITIES    PERCENT OF TOTAL                               STOCK PRICE APPRECIATION FOR
                              UNDERLYING    OPTIONS GRANTED    EXERCISE OR                        OPTION TERM(2)
                                OPTIONS       TO EMPLOYEES     BASE PRICE    EXPIRATION   ------------------------------
            NAME              GRANTED(1)     IN FISCAL YEAR     PER SHARE       DATE           5%               10%
            ----              -----------   ----------------   -----------   ----------   -------------    -------------
<S>                           <C>           <C>                <C>           <C>          <C>              <C>
John R. Bertucci............        --              --               --            --             --               --
Ronald C. Weigner...........    60,000            9.47%           $6.67        7/9/08       $251,684         $637,816
Joseph A. Maher, Jr. .......    60,000            9.47             6.67        7/9/08        251,684          637,816
William D. Stewart..........    60,000            9.47             6.67        7/9/08        251,684          637,816
Leo Berlinghieri............    60,000            9.47             6.67        7/9/08        251,684          637,816
</TABLE>
 
- ---------------
(1) These options become exercisable with respect to 20% of the shares granted
    on July 9, 1999 and with respect to the remainder of the shares on a
    quarterly basis during the following four years.
 
(2) Amounts represent hypothetical gains that could be achieved for the
    respective options if exercised at the end of the option term. These gains
    are based on assumed rates of stock price appreciation of 5% and 10%
    compounded annually from the date the respective options were granted to
    their expiration date. These numbers are calculated based on rules
    promulgated by the Securities and Exchange Commission and do not reflect
    MKS's estimate of future stock price growth. Actual gains, if any, on stock
    option exercises and common stock are dependent on the timing of such
    exercise and the future performance of the common stock.
 
OPTION EXERCISES AND HOLDINGS
 
     The following table sets forth information concerning option exercises and
option holdings for the fiscal year ended December 31, 1998 with respect to each
of the Named Executive Officers.
 
                      AGGREGATED OPTION EXERCISES IN 1998
                           AND YEAR-END OPTION VALUES
 
<TABLE>
<CAPTION>
                                                   NUMBER OF SHARES
                                                      UNDERLYING                 VALUE OF UNEXERCISED
                                                 UNEXERCISED OPTIONS             IN-THE-MONEY OPTIONS
                                                     AT YEAR-END                    AT YEAR-END(1)
                                             ----------------------------    ----------------------------
                   NAME                      EXERCISABLE    UNEXERCISABLE    EXERCISABLE    UNEXERCISABLE
                   ----                      -----------    -------------    -----------    -------------
<S>                                          <C>            <C>              <C>            <C>
John R. Bertucci...........................        --               --              --               --
Ronald C. Weigner..........................    75,961          110,639        $757,331       $  968,671
Joseph A. Maher, Jr. ......................    44,310          142,290         441,771        1,284,231
William D. Stewart.........................    75,961          110,639         757,331          968,671
Leo Berlinghieri...........................    75,961          110,639         757,331          968,671
</TABLE>
 
- ---------------
(1) Values are based on the difference between the fair market value of the
    underlying shares at December 31, 1998 ($14.40 per share) and the exercise
    price of each option listed (between $4.43 and $6.67 per share).
 
                                       48
<PAGE>   50
 
STOCK PLANS
 
  1995 Stock Incentive Plan
 
     MKS's Amended and Restated 1995 Stock Incentive Plan (the "1995 Stock
Plan") provides for the grant of incentive stock options, nonstatutory stock
options, stock appreciation rights, performance shares and awards of restricted
stock and unrestricted stock. An aggregate of 3,750,000 shares of common stock
may be issued pursuant to the 1995 Stock Plan (subject to adjustment for certain
changes in MKS's capitalization). No award may be made under the 1995 Stock Plan
after November 30, 2005.
 
     The 1995 Stock Plan is administered by the Board of Directors and the
Compensation Committee. The Board of Directors has the authority to grant awards
under the 1995 Stock Plan and to accelerate, waive or amend certain provisions
of outstanding awards. The Board of Directors has authorized the Compensation
Committee to administer certain aspects of the 1995 Stock Plan and has
authorized the Chief Executive Officer of MKS to grant awards to non-executive
officer employees. The maximum number of shares represented by such awards may
not exceed 450,000 shares in the aggregate or 30,000 shares to any one employee.
 
     Incentive Stock Options and Nonstatutory Options.  Optionees receive the
right to purchase a specified number of shares of common stock at some time in
the future at an option price and subject to such terms and conditions as are
specified at the time of the grant. Incentive stock options and options that the
Board of Directors or Compensation Committee intends to qualify as
performance-based compensation under Section 162(m) of the Internal Revenue Code
may not be granted at an exercise price less than the fair market value of the
common stock on the date of grant (or less than 110% of the fair market value in
the case of incentive stock options granted to optionees holding 10% or more of
the voting stock of MKS). All other options may be granted at an exercise price
that may be less than, equal to or greater than the fair market value of the
common stock on the date of grant.
 
     Stock Appreciation Rights and Performance Shares.  A stock appreciation
right is based on the value of common stock and entitles the holder to receive
consideration to the extent that the fair market value on the date of exercise
of the shares of common stock underlying the right exceeds the fair market value
of the underlying shares on the date the right was granted. A performance share
award entitles the recipient to acquire shares of common stock upon the
attainment of specified performance goals.
 
     Restricted and Unrestricted Stock.  Restricted stock awards entitle
recipients to acquire shares of common stock, subject to the right of MKS to
repurchase all or part of such shares at their purchase price from the recipient
in the event that the conditions specified in the applicable stock award are not
satisfied prior to the end of the applicable restriction period established for
such award. MKS may also grant (or sell at a purchase price not less than 85% of
the fair market value on the date of such sale) to participants shares of common
stock free of any restrictions under the 1995 Stock Plan.
 
     All of the employees, officers, directors, consultants and advisors of MKS
and its subsidiaries who are expected to contribute to MKS's future growth and
success are eligible to participate in the 1995 Stock Plan.
 
     Section 162(m) of the Internal Revenue Code disallows a tax deduction to
public companies for certain compensation in excess of $1.0 million paid to a
company's chief executive officer or to any of the four other most highly
compensated executive officers. Certain compensation, including "performance-
based compensation," is not included in compensation subject to the $1.0 million
limitation. The 1995 Stock Plan limits to 1,350,000 the maximum number of shares
of common stock with respect to which awards may be granted to any employee in
any calendar year. This limitation is intended to preserve the tax deductions to
MKS that might otherwise be unavailable under Section 162(m) with respect to
certain awards.
 
     Prior to the date of this prospectus, MKS plans to grant options (to vest
20% after one year and 5% per quarter thereafter) to purchase approximately
350,000 shares of common stock to certain employees of MKS, at an exercise price
equal to the initial public offering price.
 
                                       49
<PAGE>   51
 
  1999 Employee Stock Purchase Plan
 
     MKS's 1999 Employee Stock Purchase Plan (the "Purchase Plan") authorizes
the issuance of up to an aggregate of 450,000 shares of common stock to
participating employees. MKS will make one or more offerings to employees to
purchase common stock under the Purchase Plan. Offerings under the Purchase Plan
commence on June 1 and December 1 and terminate, respectively on November 30 and
May 31. During each offering, the maximum number of shares which may be
purchased by a participating employee is determined on the first day of this
offering period under a formula whereby 85% of the market value of a share of
common stock on the first day of this offering period is divided into an amount
equal to 10% of the employee's annualized compensation (or such lower percentage
as may be established by the Compensation Committee) for the immediately
preceding six-month period. An employee may elect to have up to 10% deducted
from his or her regular salary (or such lower percentage as may be established
by the Compensation Committee) for this purpose. The price at which an
employee's option is exercised is the lower of (1) 85% of the closing price of
the common stock on the Nasdaq National Market on the day that this offering
commences or (2) 85% of the closing price on the day that this offering
terminates.
 
     The Purchase Plan is administered by the Board of Directors and the
Compensation Committee. With certain exceptions, all eligible employees,
including directors and officers, regularly employed by MKS for at least six
months on the applicable offering commencement date are eligible to participate
in the Purchase Plan. The Purchase Plan is intended to qualify as an "employee
stock purchase plan" as defined in Section 423 of the Internal Revenue Code.
 
  1997 Director Stock Option Plan
 
     MKS's 1997 Director Stock Option Plan (the "1997 Director Plan") authorizes
the issuance of up to an aggregate of 300,000 shares of common stock. The 1997
Director Plan is administered by MKS's Board of Directors. Options are granted
under the 1997 Director Plan only to directors of MKS who are not employees of
MKS. Under the 1997 Director Plan, prior to the date of this prospectus each
existing eligible director will receive an option to purchase 10,500 shares of
common stock at an exercise price equal to the initial public offering price and
future non-employee directors will receive an option to purchase 11,250 shares
of common stock upon their initial election to the Board of Directors. Each
initial option will vest over a three-year period in 12 equal quarterly
installments following the date of grant. On the date of each annual meeting of
the stockholders, options will be automatically granted to each eligible
director who has been in office for at least six months prior to the date of the
annual meeting of the stockholders. Each annual option will entitle the holder
to purchase 6,000 shares of common stock. Each annual option will become
exercisable on the day prior to the first annual meeting of stockholders
following the date of grant, or if no such meeting is held within 13 months
after the date of grant, on the 13-month anniversary of the date of grant. The
exercise price of all options granted under the 1997 Director Plan is equal to
the fair market value of the common stock on the date of grant. Options granted
under the 1997 Director Plan terminate upon the earlier of three months after
the optionee ceases to be a director of MKS or ten years after the grant date.
In the event of a change in control of MKS, the vesting of all options then
outstanding would be accelerated in full and any restrictions on exercising
outstanding options would terminate.
 
     The Company's 1996 Director Stock Option Plan, under which options have
been granted to, and may still be exercised by, four non-employee directors of
MKS, has been terminated. See "-- Director Compensation."
 
                                       50
<PAGE>   52
 
  Employment Agreements
 
     MKS entered into an employment agreement with each of Messrs. Stewart,
Maher, Berlinghieri and Weigner.
 
     Each agreement sets a base salary for each employee which is reviewed
annually. In addition to a base salary, each employee is entitled, under MKS's
Management Incentive Program, to a bonus equal to a percentage of his base
salary if MKS attains specified financial goals during the year. Each employee
is also entitled to standard benefits including:
 
     - participation in a profit sharing and retirement savings plan
 
     - vacation days
 
     - life insurance
 
     - medical/dental insurance
 
     The remaining provisions of each agreement are also substantially the same.
 
     The term of employment for each is from month to month with termination:
 
     - upon the death of the employee
 
     - at the election of MKS if the employee fails or refuses to perform
 
     - at the election of MKS if the employee commits any acts not in MKS's best
       interest
 
     Payment by MKS upon termination depends on how employment is terminated:
 
     - if employment is terminated after the expiration of a 30 day notice
       period, MKS has no further obligation for compensation
 
     - if employment is terminated by death, MKS must pay the employee's estate
       the compensation owed to him at the end of the month of his death
 
     - if employment is terminated at the election of MKS, MKS must pay the
       employee through the last day of actual employment
 
     Each of the agreements contains non-competition provisions during the term
of employment and for the period one year after termination of employment. Under
these provisions, Messrs. Stewart, Maher, Berlinghieri and Weigner may not:
 
     - engage in any competitive business or activity
 
     - for the 12 months subsequent to termination, work for, employ, become a
       partner with, or cause to be employed any employee, officer or agent of
       MKS
 
     - for the 12 months subsequent to termination, give, sell or lease any
       competitive services or goods to any customer of MKS
 
     - have any financial interest in or be a director, officer, stockholder,
       partner, employee or consultant to any competitor of MKS
 
                                       51
<PAGE>   53
 
                              CERTAIN TRANSACTIONS
 
     Mr. Chute, a director of MKS, MKS's clerk, and a co-trustee of certain of
the Bertucci Family Trusts (see "Principal Stockholders") and Mr. Thomas H.
Belknap, a co-trustee of certain of the Bertucci Family trusts, are attorneys at
the law firm of Hill & Barlow, a professional corporation. Hill & Barlow has
provided legal services to MKS during the calendar year ended December 31, 1998
for which it was compensated by MKS in the aggregate amount of $183,000.
 
     Mr. Stewart, Corporate Vice President and General Manager of Vacuum
Products, is the general partner of Aspen Industrial Park Partnership. On
October 12, 1989, MKS entered into a lease with Aspen, which has been
periodically extended, for certain facilities occupied by MKS's Vacuum Products
group in Boulder, Colorado. MKS currently pays Aspen approximately $350,000
annually to lease such facilities.
 
   
     MKS has been treated as an S corporation for federal income tax purposes
since July 1, 1987. As a result, MKS currently pays no federal, and certain
state, income tax and all of the earnings of MKS are subject to federal, and
certain state, income taxation directly at the stockholder level. MKS's S
corporation status will terminate upon the closing of this offering, at which
time MKS will become subject to corporate income taxation under Subchapter C of
the Internal Revenue Code. In 1997 and 1998, MKS distributed $12.4 million and
$6.2 million, respectively, of undistributed S corporation earnings to its
stockholders. As soon as practicable following the closing of this offering, MKS
intends to make a distribution to the stockholders of record on the day prior to
the effective date of the registration statement in the amount of $40.0 million,
which is the estimated balance of the accumulated adjustments account as of the
day prior to the closing of this offering, subject to adjustment. See "S
Corporation and Termination of S Corporation Status."
    
 
     MKS believes that the transactions listed above were made on terms no less
favorable to the Company than could have been obtained from unaffiliated third
parties. Commencing on the effective date of this offering, all future
transactions between MKS and its officers, directors or other affiliates must
(1) be approved by a majority of the members of the Board of Directors and a
majority of the disinterested members of the Board; and (2) be on terms no less
favorable to MKS than could be obtained from unaffiliated third parties.
 
                                       52
<PAGE>   54
 
                       PRINCIPAL AND SELLING STOCKHOLDERS
 
     The following table sets forth certain information regarding beneficial
ownership of MKS's common stock as of December 31, 1998, and as adjusted to
reflect the sale of shares offered hereby, by (1) each of the directors of MKS,
(2) each of the Named Executive Officers, (3) each person known to MKS to own
beneficially more than 5% of MKS's common stock and (4) all directors and
executive officers as a group.
 
     Unless otherwise indicated, each person named in the table has sole voting
power and investment power or shares such power with his or her spouse with
respect to all shares of capital stock listed as owned by such person.
Beneficial ownership is determined in accordance with the rules of the
Securities and Exchange Commission and includes voting or investment power with
respect to the securities. The number of shares of common stock outstanding used
in calculating the percentage for each listed person includes any shares the
individual has the right to acquire within 60 days of December 31, 1998.
 
     All of the shares being offered by the selling stockholders are owned by
trusts for the benefit of Mr. Bertucci and members of his family.
 
<TABLE>
<CAPTION>
                                                     SHARES                              SHARES
                                               BENEFICIALLY OWNED      NUMBER      BENEFICIALLY OWNED
                                                PRIOR TO OFFERING        OF          AFTER OFFERING
                                              ---------------------    SHARES     ---------------------
          NAME OF BENEFICIAL OWNER              NUMBER      PERCENT    OFFERED      NUMBER      PERCENT
          ------------------------            ----------    -------    -------    ----------    -------
<S>                                           <C>           <C>        <C>        <C>           <C>
John R. Bertucci............................  17,261,915(1)  95.6%     500,000    16,761,915     69.7%
Ronald C. Weigner...........................      82,291(2)     *           --        82,291        *
John J. Sullivan............................     614,010(3)   3.4           --       614,010      2.6
Joseph A. Maher, Jr.........................      44,310(2)     *           --        44,310        *
William D. Stewart..........................      82,291(2)     *           --        82,291        *
Leo Berlinghieri............................      82,291(2)     *           --        82,291        *
Richard S. Chute............................   2,766,852(4)  15.3      300,000     2,466,852     10.3
Owen W. Robbins.............................       8,027(2)     *           --         8,027        *
Robert J. Therrien..........................       8,027(2)     *           --         8,027        *
Louis P. Valente............................       8,027(2)     *           --         8,027        *
Thomas H. Belknap...........................   2,331,902(5)  12.9      200,000     2,131,902      8.9
All executive officers and directors as a
  group.....................................  18,199,216     99.0%     500,000    17,699,216     72.6%
</TABLE>
 
- ---------------
 *  Less than 1% of outstanding common stock.
 
(1) Includes 6,046,208 shares held directly by Mr. Bertucci, 6,124,980 shares
    held directly by Mr. Bertucci's wife, and 5,090,727 shares held by Bertucci
    family trusts for which either Mr. or Mrs. Bertucci serves as a co-trustee.
 
(2) Comprised solely of options exercisable within 60 days of December 31, 1998.
 
(3) Includes 316,500 shares held in a grantor retained annuity trust.
 
(4) Includes 2,758,825 shares held by certain of the Bertucci family trusts for
    which Mr. Chute serves as a co-trustee and 8,027 shares subject to options
    held by Mr. Chute exercisable within 60 days of December 31, 1998.
 
(5) Represents shares held by certain of the Bertucci family trusts for which
Mr. Belknap serves as a
     co-trustee.
 
                                       53
<PAGE>   55
 
                          DESCRIPTION OF CAPITAL STOCK
 
     The authorized capital stock of MKS will consist of 50,000,000 shares of
common stock, no par value per share, and 2,000,000 shares of preferred stock,
$.01 par value per share, after giving effect to the amendment and restatement
of MKS's Restated Articles of Organization which will be filed with the
Secretary of State of The Commonwealth of Massachusetts prior to the closing of
this offering.
 
COMMON STOCK
 
     As of December 31, 1998, there were 18,053,167 shares of common stock
outstanding and held of record by twenty-three stockholders.
 
     Upon the closing of this offering, all holders of common stock shall be
entitled to one vote for each share held on all matters submitted to a vote of
stockholders and will not have cumulative voting rights. Accordingly, holders of
a majority of the shares of common stock entitled to vote in any election of
directors may elect all of the directors standing for election. Holders of
common stock are entitled to receive ratably such dividends, if any, as may be
declared by the Board of Directors out of funds legally available therefor,
subject to any preferential dividend rights of any outstanding preferred stock.
Upon the liquidation, dissolution or winding up of MKS, the holders of common
stock are entitled to receive ratably the net assets of MKS available after the
payment of all debts and other liabilities, subject to the prior rights of any
outstanding preferred stock. Holders of the common stock have no preemptive,
subscription, redemption or conversion rights. The outstanding shares of common
stock are, and the shares offered by MKS in this offering made by this
prospectus will be, when issued and paid for, fully paid and nonassessable. The
rights, preferences and privileges of holders of common stock are subject to,
and may be adversely affected by, the rights of the holders of shares of any
series of preferred stock that MKS may designate and issue in the future. There
are no shares of preferred stock outstanding.
 
PREFERRED STOCK
 
     The Articles of Organization authorize the Board of Directors, subject to
certain limitations prescribed by law, without further stockholder approval,
from time to time to issue up to an aggregate of 2,000,000 shares of preferred
stock in one or more series and to fix or alter the designations, preferences
and rights, and any qualifications, limitations or restrictions thereof, of the
shares of each such series, including the number of shares constituting any such
series and the dividend rights, dividend rates, conversion rights, voting
rights, terms of redemption (including sinking fund provisions), redemption
price or prices and liquidation preferences thereof. The issuance of preferred
stock may have the effect of delaying, deferring or preventing a change in
control of MKS. MKS has no present plans to issue any shares of preferred stock.
 
MASSACHUSETTS LAW AND CERTAIN PROVISIONS OF MKS'S RESTATED ARTICLES OF
ORGANIZATION AND BY-LAWS
 
     MKS intends to amend and restate its By-Laws prior to the closing of this
offering. The By-Laws will include a provision excluding MKS from the
applicability of Massachusetts General Laws Chapter 110D, entitled "Regulation
of Control Share Acquisitions." In general, this statute provides that any
stockholder of a corporation subject to this statute who acquires 20% or more of
the outstanding voting stock of a corporation may not vote such stock unless the
stockholders of the corporation so authorize. The Board of Directors will be
able to amend the By-Laws at any time to subject MKS to this statute
prospectively.
 
     Massachusetts General Laws Chapter 156B, Section 50A generally requires
that publicly-held Massachusetts corporations have a classified board of
directors consisting of three classes as nearly equal in size as possible,
unless the corporation elects to opt out of the statute's coverage. The By-Laws
will contain provisions which give effect to Section 50A.
 
     The By-Laws will require that nominations for the Board of Directors made
by a stockholder of a planned nomination must be given not less than 30 and not
more than 90 days prior to a scheduled meeting, provided that if less than 40
days' notice is given of the date of the meeting, a stockholder will
 
                                       54
<PAGE>   56
 
have ten days within which to give such notice. The stockholder's notice of
nomination must include particular information about the stockholder, the
nominee and any beneficial owner on whose behalf the nomination is made. MKS may
require any proposed nominee to provide such additional information as is
reasonably required to determine the eligibility of the proposed nominee.
 
     The By-Laws will also require that a stockholder seeking to have any
business conducted at a meeting of stockholders give notice to MKS not less than
60 and not more than 90 days prior to the scheduled meeting, provided in certain
circumstances that a ten-day notice rule applies. The notice from the
stockholder will be required to describe the proposed business to be brought
before the meeting and include information about the stockholder making the
proposal, any beneficial owner on whose behalf the proposal is made, and any
other stockholder known to be supporting the proposal. The By-Laws will require
MKS to call a special stockholders meeting at the request of stockholders
holding at least 40% of the voting power of MKS.
 
     The Articles of Organization will provide that the directors and officers
of MKS shall be indemnified by MKS to the fullest extent authorized by
Massachusetts law, as it now exists or may in the future be amended, against all
expenses and liabilities reasonably incurred in connection with service for or
on behalf of MKS. In addition, the Articles of Organization will provide that
the directors of MKS will not be personally liable for monetary damages to MKS
for breaches of their fiduciary duty as directors, unless they violated their
duty of loyalty to MKS or its stockholders, acted in bad faith, knowingly or
intentionally violated the law, which could include securities laws, authorized
illegal dividends or redemptions or derived an improper personal benefit from
their action as directors.
 
     The Articles of Organization will provide that any amendment to the
Articles of Organization, the sale, lease or exchange of all or substantially
all of MKS's property and assets, or the merger or consolidation of MKS into or
with any corporation may be authorized by the approval of the holders of a
majority of the shares of each class of stock entitled to vote thereon, rather
than by two-thirds as otherwise provided by statute, provided that the
transactions have been authorized by a majority of the members of the Board of
Directors and the requirements of any other applicable provisions of the
Articles of Organization have been met.
 
     The Articles of Organization will contain a provision excluding MKS from
the applicability of Massachusetts General Laws Chapter 110F, entitled "Business
Combinations with Interested Shareholders." In general, Chapter 110F places
limitations on a Massachusetts corporation's ability to engage in business
combinations with certain stockholders for a period of three years, unless the
corporation elects to opt out of the statute's coverage by including such a
provision in its Articles of Organization.
 
TRANSFER AGENT AND REGISTRAR
 
     The Transfer Agent and Registrar for the common stock is BankBoston, N.A.
 
                                       55
<PAGE>   57
 
                        SHARES ELIGIBLE FOR FUTURE SALE
 
     Prior to this offering, there has been no public market for the securities
of MKS. Upon completion of this offering, based upon the number of shares
outstanding at December 31, 1998, there will be        shares of common stock of
MKS outstanding assuming the underwriters do not exercise their over-allotment
option, and no options are exercised. Of these shares, the 6,500,000 shares sold
in this offering will be freely tradable without restriction or further
registration under the Securities Act, except that any shares purchased by
"affiliates" of MKS, as that term is defined in Rule 144 under the Securities
Act, may generally only be sold in compliance with the limitations of Rule 144
described below.
 
SALES OF RESTRICTED SHARES
 
     The outstanding shares of common stock not sold in this offering will be
deemed "restricted securities" under Rule 144 under the Securities Act. Of these
shares, 17,553,165 are subject to 180-day lock-up agreements with the
representatives. Upon expiration of the lock-up agreements 180 days after the
date of this prospectus, all such shares will be available for sale in the
public market, subject to the provisions of Rule 144.
 
     Stockholders who are parties to the lock-up agreement have agreed that for
a period of 180 days after the date of this prospectus, they will not sell,
offer, contract or grant any option to sell, pledge, transfer, establish an open
put equivalent position or otherwise dispose of any shares of common stock, any
options to purchase shares of common stock or any shares convertible into or
exchangeable for shares of common stock, owned directly by such persons or with
respect to which they have the power of disposition, without the prior written
consent of NationsBanc Montgomery Securities LLC.
 
     In general, under Rule 144, beginning 90 days after the effective date of
this prospectus, a stockholder who has beneficially owned his or her restricted
securities for at least one year will be entitled to sell, within any
three-month period, a limited number of such shares. The number of shares may
not exceed the greater of 1% of the then outstanding shares of common stock or
the average weekly trading volume in the common stock during the four preceding
calendar weeks. In addition, under Rule 144(k), if a period of at least two
years has elapsed since the date restricted securities were acquired from MKS, a
stockholder who is not an affiliate of MKS at the time of sale and has not been
an affiliate of MKS for at least three months prior to the sale will be entitled
to sell the shares immediately without restriction.
 
     Securities issued in reliance on Rule 701, such as shares of common stock
acquired upon exercise of certain options granted under MKS's stock plans, are
also restricted and, beginning 90 days after the effective date of this
prospectus, may be sold by stockholders other than affiliates of MKS subject
only to the manner of sale provisions of Rule 144 and by affiliates under Rule
144 without compliance with its one-year holding period requirement.
 
OPTIONS
 
     As of December 31, 1998 there were options outstanding to purchase an
aggregate of 2,132,575 shares of MKS's common stock, of which options to
purchase an aggregate of 804,701 shares were exercisable. Of these, 802,009
shares were subject to lock-up agreements. The option to purchase the remaining
2,692 shares has since expired. MKS intends to file registration statements on
Form S-8 under the Securities Act to register all shares of common stock
issuable under each of the 1995 Stock Plan, Purchase Plan, the 1997 Director
Plan and the 1996 Director Stock Option Plan promptly following the consummation
of this offering. Shares issued pursuant to such plans shall be, after the
effective date of the Form S-8 registration statements, eligible for resale in
the public market without restriction, subject to Rule 144 limitations
applicable to affiliates and the lock-up agreements noted above, if applicable.
 
                                       56
<PAGE>   58
 
                                  UNDERWRITING
 
     MKS is offering the shares of common stock described in this prospectus
through a number of underwriters. NationsBanc Montgomery Securities LLC,
Donaldson, Lufkin & Jenrette Securities Corporation and Lehman Brothers Inc. are
the representatives of the underwriters. MKS and the selling stockholders have
entered into an underwriting agreement with the representatives. Subject to the
terms and conditions of the underwriting agreement, MKS and the selling
stockholders have agreed to sell to the underwriters, and the underwriters have
each agreed to purchase, the number of shares of common stock listed next to its
name in the following table.
 
<TABLE>
<CAPTION>
                                                              NUMBER OF
                        UNDERWRITER                            SHARES
                        -----------                           ---------
<S>                                                           <C>
NationsBanc Montgomery Securities LLC.......................
Donaldson, Lufkin & Jenrette Securities Corporation.........
Lehman Brothers Inc. .......................................
                                                              ---------
          Total.............................................  6,500,000
                                                              =========
</TABLE>
 
     The underwriters initially will offer shares to the public at the price
specified on the cover page of this prospectus. The underwriters may allow to
some dealers a concession of not more than $      per share. The underwriters
also may allow, and any other dealers may reallow, a concession of not more than
$
per share to some other dealers. If all the shares are not sold at the initial
public offering price, the underwriters may change the offering price and the
other selling terms. The common stock is offered subject to a number of
conditions, including:
 
     - receipt and acceptance of our common stock by the underwriters
 
     - the right to reject orders in whole or in part
 
     MKS has granted an option to the underwriters to buy up to 975,000
additional shares of common stock. These additional shares would cover sales of
shares by the underwriters which exceed the number of shares specified in the
table above. The underwriters have 30 days to exercise this option. If the
underwriters exercise this option, they will each purchase additional shares
approximately in proportion to the amounts specified in the table above.
 
     MKS and all holders of its stock prior to this offering, as well as most
holders of stock options, have entered into lock-up agreements with the
underwriters. Under those agreements, MKS and those holders of stock and options
may not dispose of or hedge any MKS common stock or securities convertible into
or exchangeable for shares of MKS common stock. These restrictions will be in
effect for a period of 180 days after the date of this prospectus. At any time
and without notice, NationsBanc Montgomery Securities LLC may, in its sole
discretion, release all or some of the securities from these lock-up agreements.
 
     MKS and the selling stockholders will indemnify the underwriters against
some liabilities, including some liabilities under the Securities Act. If MKS is
unable to provide this indemnification, MKS and the selling stockholders will
contribute to payments the underwriters may be required to make in respect of
those liabilities.
 
     In connection with this offering, the underwriters may purchase and sell
shares of common stock in the open market. These transactions may include:
 
     - short sales
 
     - stabilizing transactions
 
     - purchases to cover positions created by short sales
 
     Short sales involve the sale by the underwriters of a greater number of
shares than they are required to purchase in this offering. Stabilizing
transactions consist of bids or purchases made for the purpose of
 
                                       57
<PAGE>   59
 
preventing or retarding a decline in the market price of the common stock while
this offering is in progress.
 
     The underwriters also may impose a penalty bid. This means that if the
representatives purchase shares in the open market in stabilizing transactions
or to cover short sales, the representatives can require the underwriters that
sold those shares as part of this offering to repay the underwriting discount
received by them.
 
     The underwriters may engage in activities that stabilize, maintain or
otherwise affect the price of the common stock, including:
 
     - over-allotment
 
     - stabilization
 
     - syndicate covering transactions
 
     - imposition of penalty bids
 
     As a result of these activities, the price of the common stock may be
higher than the price that otherwise might exist in the open market. If the
underwriters commence these activities, they may discontinue them at any time.
The underwriters may carry out these transactions on the Nasdaq National Market,
in the over-the-counter market or otherwise.
 
     The underwriters do not expect sales to discretionary accounts to exceed 5%
of the total number of shares of common stock offered by this prospectus.
 
     Prior to this offering, there has been no public market for the common
stock of MKS. The initial public offering price will be negotiated among MKS,
the selling stockholders and the underwriters. Among the factors to be
considered in such negotiations are:
 
     - the history of, and prospects for, MKS and the industry in which it
       competes
 
     - the past and present financial performance of MKS
 
     - an assessment of MKS's management
 
     - the present state of MKS's development
 
     - the prospects for future earnings of MKS
 
     - the prevailing market conditions of the applicable U.S. securities market
at the time of this offering
 
     - market valuations of publicly traded companies that MKS and the
       representatives believe to be comparable to MKS
 
     - other factors deemed relevant
 
                                 LEGAL MATTERS
 
     The validity of the common stock offered hereby will be passed upon for MKS
by Hale and Dorr LLP, Boston, Massachusetts. Certain legal matters in connection
with this offering will be passed upon for the underwriters by Ropes & Gray,
Boston, Massachusetts.
 
                                    EXPERTS
 
     The consolidated balance sheets of MKS Instruments, Inc. at December 31,
1997 and 1998 and the consolidated statements of income, stockholders' equity
and cash flows for each of the three years in the period ended December 31, 1998
included in this prospectus have been included herein in reliance on the report
of PricewaterhouseCoopers LLP, independent accountants, given upon the authority
of that firm as experts in accounting and auditing.
 
                                       58
<PAGE>   60
 
                             ADDITIONAL INFORMATION
 
     MKS has filed with the Securities and Exchange Commission, a registration
statement on Form S-1 under the Securities Act with respect to the common stock
offered hereby. This prospectus, which constitutes part of the registration
statement, does not contain all of the information set forth in the registration
statement, certain parts of which are omitted in accordance with the rules and
regulations of the Securities and Exchange Commission. For further information
with respect to MKS and the common stock offered hereby, reference is made to
the registration statement. Statements contained in this prospectus as to the
contents of any contract or other document filed as an exhibit to the
registration statement are not necessarily complete, and in each instance
reference is made to the copy of such document filed as an exhibit to the
registration statement, each such statement being qualified in all respects by
such reference. The registration statement (and all amendments, exhibits and
schedules thereto) may be inspected without charge at the principal office of
the Securities and Exchange Commission in Washington, D.C. and copies of all or
any part of which may be inspected and copied at the public reference facilities
maintained by the Securities and Exchange Commission at 450 Fifth Street, N.W.,
Judiciary Plaza, Room 1024, Washington, D.C. 20549, and at the Securities and
Exchange Commission's regional offices located at Citicorp Center, 500 West
Madison Street, Suite 1400, Chicago, Illinois 60661-2511 and 7 World Trade
Center, Suite 1300, New York, New York 10048. Copies of such material can also
be obtained at prescribed rates by mail from the Public Reference Section of the
Securities and Exchange Commission at 450 Fifth Street, N.W., Washington, D.C.
20549. In addition, the Securities and Exchange Commission maintains a website
(http://www.sec.gov) that contains reports, proxy and information statements and
other information regarding registrants that file electronically with the
Securities and Exchange Commission.
 
     MKS intends to distribute to its stockholders annual reports containing
audited consolidated financial statements.
 
                                       59
<PAGE>   61
 
                             MKS INSTRUMENTS, INC.
 
                   INDEX TO CONSOLIDATED FINANCIAL STATEMENTS
 
<TABLE>
<CAPTION>
                                                              PAGE
                                                              ----
<S>                                                           <C>
Report of Independent Accountants...........................  F-2
Consolidated Balance Sheets at December 31, 1997 and
  1998......................................................  F-3
Consolidated Statements of Income for the Years Ended
  December 31, 1996, 1997, and 1998.........................  F-4
Consolidated Statements of Stockholders' Equity for the
  Years Ended December 31, 1996, 1997, and 1998.............  F-5
Consolidated Statements of Cash Flows for the Years Ended
  December 31, 1996, 1997, and 1998.........................  F-6
Notes to Consolidated Financial Statements..................  F-7
</TABLE>
 
                                       F-1
<PAGE>   62
 
                       REPORT OF INDEPENDENT ACCOUNTANTS
 
To the Board of Directors and Stockholders of
  MKS Instruments, Inc.:
 
     In our opinion, the accompanying consolidated balance sheets and related
consolidated statements of income, stockholders' equity and cash flows present
fairly, in all material respects, the financial position of MKS Instruments,
Inc. and its subsidiaries at December 31, 1997 and 1998 and the results of their
operations and their cash flows for each of the three years in the period ended
December 31, 1998, in conformity with generally accepted accounting principles.
These financial statements are the responsibility of the Company's management;
our responsibility is to express an opinion on these financial statements based
on our audits. We conducted our audits of these statements in accordance with
generally accepted auditing standards which require that we plan and perform the
audit to obtain reasonable assurance about whether the financial statements are
free of material misstatement. An audit includes examining, on a test basis,
evidence supporting the amounts and disclosures in the financial statements,
assessing the accounting principles used and significant estimates made by
management, and evaluating the overall financial statement presentation. We
believe that our audits provide a reasonable basis for the opinion expressed
above.
 
                                            PRICEWATERHOUSECOOPERS LLP
 
Boston, Massachusetts
January 22, 1999, except for the
information in the first and second
paragraph of Note 13 as to which the date
is January 28, 1999 and February 24, 1999,
respectively
 
                                       F-2
<PAGE>   63
 
                             MKS INSTRUMENTS, INC.
 
                          CONSOLIDATED BALANCE SHEETS
                       (IN THOUSANDS, EXCEPT SHARE DATA)
 
<TABLE>
<CAPTION>
                                                                              DECEMBER 31, 1998
                                                            DECEMBER 31,    ----------------------
                                                                1997        ACTUAL      PRO FORMA
                                                            ------------    -------    -----------
                                                                                        (NOTE 2)
                                                                                       (UNAUDITED)
<S>                                                         <C>             <C>        <C>
                                              ASSETS
Current assets:
     Cash and cash equivalents............................    $  2,511      $11,188      $11,188
     Marketable equity securities.........................         614          538          538
     Trade accounts receivable, net of allowance for
       doubtful accounts of $610 and $656 at December 31,
       1997 and 1998, respectively........................      32,439       20,674       20,674
     Inventories..........................................      29,963       24,464       24,464
     Deferred tax asset...................................         682          698          698
     Other current assets.................................       1,670          971          971
                                                              --------      -------      -------
          Total current assets............................      67,879       58,533       58,533
     Property, plant and equipment, net...................      33,976       32,725       32,725
     Other assets.........................................       4,681        4,974        4,974
                                                              --------      -------      -------
          Total assets....................................    $106,536      $96,232      $96,232
                                                              ========      =======      =======
 
                               LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
     Short-term borrowings................................    $ 10,721      $ 9,687      $ 9,687
     Current portion of long-term debt....................       2,070        2,058        2,058
     Current portion of capital lease obligations.........       1,061        1,074        1,074
     Accounts payable.....................................       7,433        3,677        3,677
     Accrued compensation.................................       7,501        3,985        3,985
     Other accrued expenses...............................       6,883        5,280        5,280
     Income taxes payable.................................       1,889        1,279        1,279
     Distribution payable.................................          --           --       35,926
                                                              --------      -------      -------
          Total current liabilities.......................      37,558       27,040       62,966
Long-term debt............................................      13,748       12,042       12,042
Long-term portion of capital lease obligations............       1,876        1,744        1,744
Deferred tax liability....................................         133          117          117
Other liabilities.........................................         373          463          463
Commitments and contingencies (Note 7)
Stockholders' equity:
     Common Stock, Class A, no par value; 11,250,000
       shares authorized, 7,766,910 issued and
       outstanding........................................          40           40           40
     Common Stock, Class B (non voting) no par value;
       18,750,000 shares authorized; 10,286,255 and
       10,286,257 shares issued and outstanding at
       December 31, 1997 and 1998, respectively...........          73           73           73
     Additional paid-in capital...........................          48           48           48
     Retained earnings....................................      51,443       52,479       16,553
     Accumulated other comprehensive income...............       1,244        2,186        2,186
                                                              --------      -------      -------
          Total stockholders' equity......................      52,848       54,826       18,900
                                                              --------      -------      -------
          Total liabilities and stockholders' equity......    $106,536      $96,232      $96,232
                                                              ========      =======      =======
</TABLE>
 
   The accompanying notes are an integral part of the consolidated financial
                                  statements.
                                       F-3
<PAGE>   64
 
                             MKS INSTRUMENTS, INC.
 
                       CONSOLIDATED STATEMENTS OF INCOME
                     (IN THOUSANDS, EXCEPT PER SHARE DATA)
 
<TABLE>
<CAPTION>
                                                                  YEAR ENDED DECEMBER 31,
                                                              --------------------------------
                                                                1996        1997        1998
                                                              --------    --------    --------
<S>                                                           <C>         <C>         <C>
Net sales...................................................  $170,862    $188,080    $139,763
Cost of sales...............................................   102,008     107,606      83,784
                                                              --------    --------    --------
Gross profit................................................    68,854      80,474      55,979
Research and development....................................    14,195      14,673      12,137
Selling, general and administrative.........................    37,191      41,838      34,707
Restructuring...............................................     1,400          --          --
                                                              --------    --------    --------
Income from operations......................................    16,068      23,963       9,135
Interest expense............................................     2,378       2,132       1,483
Interest income.............................................        92         271         296
Other income (expense), net.................................      (479)        166         187
                                                              --------    --------    --------
Income before income taxes..................................    13,303      22,268       8,135
Provision for income taxes..................................       800       1,978         949
                                                              --------    --------    --------
Net income..................................................  $ 12,503    $ 20,290    $  7,186
                                                              ========    ========    ========
 
Historical net income per share:
     Basic..................................................  $   0.69    $   1.12    $   0.40
                                                              ========    ========    ========
     Diluted................................................  $   0.69    $   1.10    $   0.38
                                                              ========    ========    ========
Historical weighted average common shares outstanding:
     Basic..................................................    18,053      18,053      18,053
                                                              ========    ========    ========
     Diluted................................................    18,053      18,388      18,720
                                                              ========    ========    ========
Pro forma data (unaudited):
     Historical income before income taxes..................  $ 13,303    $ 22,268    $  8,135
     Pro forma provision for income taxes assuming C
       corporation tax......................................     5,055       8,462       3,091
                                                              --------    --------    --------
     Pro forma net income...................................  $  8,248    $ 13,806    $  5,044
                                                              ========    ========    ========
Pro forma net income per share:
     Basic..................................................  $   0.46    $   0.76    $   0.25
                                                              ========    ========    ========
     Diluted................................................  $   0.46    $   0.76    $   0.24
                                                              ========    ========    ========
Pro forma weighted average common shares outstanding:
     Basic..................................................    18,053      18,053      20,295
                                                              ========    ========    ========
     Diluted................................................    18,053      18,262      20,780
                                                              ========    ========    ========
</TABLE>
 
   The accompanying notes are an integral part of the consolidated financial
                                  statements.
                                       F-4
<PAGE>   65
 
                             MKS INSTRUMENTS, INC.
 
                CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY
              FOR THE YEARS ENDED DECEMBER 31, 1996, 1997 AND 1998
                       (IN THOUSANDS, EXCEPT SHARE DATA)
<TABLE>
<CAPTION>
                                                                    COMMON STOCK
                                                      ----------------------------------------
                                                           CLASS A               CLASS B         ADDITIONAL
                                                      ------------------   -------------------    PAID-IN     RETAINED
                                                       SHARES     AMOUNT     SHARES     AMOUNT    CAPITAL     EARNINGS
                                                      ---------   ------   ----------   ------   ----------   --------
<S>                                                   <C>         <C>      <C>          <C>      <C>          <C>
Balance at December 31, 1995.......................   7,766,910    $40     10,286,255    $73        $48       $45,550
Distributions to stockholders......................                                                           (14,500)
Comprehensive income:
    Net income.....................................                                                            12,503
    Other comprehensive income:
    Foreign currency translation adjustment........
    Unrealized loss on investments.................
    Comprehensive income...........................
                                                      ---------    ---     ----------    ---        ---       -------
Balance at December 31, 1996.......................   7,766,910     40     10,286,255     73         48        43,553
Distributions to stockholders......................                                                           (12,400)
Comprehensive income:
    Net income.....................................                                                            20,290
    Other comprehensive income:
    Foreign currency translation adjustment........
    Unrealized gain on investments.................
    Comprehensive income...........................
                                                      ---------    ---     ----------    ---        ---       -------
Balance at December 31, 1997.......................   7,766,910     40     10,286,255     73         48        51,443
Distributions to stockholders......................                                                            (6,150)
Issuance of common stock...........................                                 2
Comprehensive income:
    Net income.....................................                                                             7,186
    Other comprehensive income:
    Foreign currency translation adjustment........
    Unrealized loss on investments.................
    Comprehensive income...........................
                                                      ---------    ---     ----------    ---        ---       -------
Balance at December 31, 1998.......................   7,766,910    $40     10,286,257    $73        $48       $52,479
                                                      =========    ===     ==========    ===        ===       =======
 
<CAPTION>
 
                                                      ACCUMULATED
                                                         OTHER                           TOTAL
                                                     COMPREHENSIVE   COMPREHENSIVE   STOCKHOLDERS'
                                                        INCOME          INCOME          EQUITY
                                                     -------------   -------------   -------------
<S>                                                  <C>             <C>             <C>
Balance at December 31, 1995.......................     $2,681                         $ 48,392
Distributions to stockholders......................                                     (14,500)
Comprehensive income:
    Net income.....................................                     $12,503          12,503
    Other comprehensive income:
    Foreign currency translation adjustment........       (766)            (766)           (766)
    Unrealized loss on investments.................       (131)            (131)           (131)
                                                                        -------
    Comprehensive income...........................                     $11,606
                                                        ------          =======        --------
Balance at December 31, 1996.......................      1,784                           45,498
Distributions to stockholders......................                                     (12,400)
Comprehensive income:
    Net income.....................................                      20,290          20,290
    Other comprehensive income:
    Foreign currency translation adjustment........       (786)            (786)           (786)
    Unrealized gain on investments.................        246              246             246
                                                                        -------
    Comprehensive income...........................                     $19,750
                                                        ------          =======        --------
Balance at December 31, 1997.......................      1,244                           52,848
Distributions to stockholders......................                                      (6,150)
Issuance of common stock...........................
Comprehensive income:
    Net income.....................................                       7,186           7,186
    Other comprehensive income:
    Foreign currency translation adjustment........        992              992             992
    Unrealized loss on investments.................        (50)             (50)            (50)
                                                                        -------
    Comprehensive income...........................                     $ 8,128
                                                        ------          =======        --------
Balance at December 31, 1998.......................     $2,186                         $ 54,826
                                                        ======                         ========
</TABLE>
 
   The accompanying notes are an integral part of the consolidated financial
                                  statements.
 
                                       F-5
<PAGE>   66
 
                             MKS INSTRUMENTS, INC.
 
                     CONSOLIDATED STATEMENTS OF CASH FLOWS
                                 (IN THOUSANDS)
 
<TABLE>
<CAPTION>
                                                                 YEAR ENDED DECEMBER 31,
                                                              ------------------------------
                                                                1996       1997       1998
                                                              --------   --------   --------
<S>                                                           <C>        <C>        <C>
Cash flows from operating activities:
     Net income.............................................  $ 12,503   $ 20,290   $  7,186
     Adjustments to reconcile net income to net cash
       provided by operating activities:
       Depreciation and amortization of property, plant, and
          equipment.........................................     5,920      5,712      6,242
       Loss on disposal of property, plant and equipment....        --        552         48
       Deferred taxes.......................................      (277)      (145)       (32)
       Provision for doubtful accounts......................       (20)       258        253
       Forward exchange contract loss (gain) realized.......       302        132     (1,211)
       Stock option compensation............................        --         95         --
       Changes in operating assets and liabilities:
          (Increase) decrease in trade accounts
            receivable......................................     6,119    (12,509)    12,908
          (Increase) decrease in inventories................     4,145     (5,930)     6,479
          (Increase) decrease in other current assets.......     3,239     (1,261)       554
          Increase (decrease) in accrued compensation.......      (220)     2,386     (3,516)
          Increase (decrease) in other accrued expenses.....    (1,520)     3,312     (1,602)
          Increase (decrease) in accounts payable...........    (4,221)     2,638     (3,682)
          Increase (decrease) in income taxes payable.......       331      1,283       (647)
                                                              --------   --------   --------
     Net cash provided by operating activities..............    26,301     16,813     22,980
                                                              --------   --------   --------
     Cash flows from investing activities:
       Purchases of property, plant and equipment...........    (9,417)    (3,269)    (3,137)
       Proceeds from sale of property, plant and
          equipment.........................................        --        203         60
       Increase in other assets.............................      (443)      (123)      (270)
       Cash received (used) to settle forward exchange
          contracts.........................................      (302)      (132)     1,211
                                                              --------   --------   --------
     Net cash used in investing activities..................   (10,162)    (3,321)    (2,136)
                                                              --------   --------   --------
     Cash flows from financing activities:
       Net (payments) borrowings on demand notes payable....       224     (1,875)        --
       Proceeds from short-term borrowings..................    11,025     24,110     15,242
       Payments on short-term borrowings....................    (9,628)   (22,938)   (17,569)
       Proceeds from long-term debt.........................       400         --         --
       Principal payments on long-term debt.................    (2,093)    (2,217)    (2,057)
       Cash distributions to stockholders...................   (14,500)   (12,400)    (6,150)
       Principal payments under capital lease obligations...      (982)      (870)    (1,257)
                                                              --------   --------   --------
     Net cash used in financing activities..................   (15,554)   (16,190)   (11,791)
                                                              --------   --------   --------
     Effect of exchange rate changes on cash and cash
       equivalents..........................................      (420)     1,394       (376)
                                                              --------   --------   --------
     Increase (decrease) in cash and cash equivalents.......       165     (1,304)     8,677
     Cash and cash equivalents at beginning of period.......     3,650      3,815      2,511
                                                              --------   --------   --------
     Cash and cash equivalents at end of period.............  $  3,815   $  2,511   $ 11,188
                                                              ========   ========   ========
     Supplemental disclosure of cash flow information:
       Cash paid during the period for:
          Interest..........................................  $  2,363   $  2,030   $  1,526
                                                              ========   ========   ========
          Income taxes......................................  $    770   $  1,078   $  1,608
                                                              ========   ========   ========
       Noncash transactions during the period:
          Equipment acquired under capital leases...........  $  2,074   $    145   $  1,138
                                                              ========   ========   ========
</TABLE>
 
   The accompanying notes are an integral part of the consolidated financial
                                  statements.
                                       F-6
<PAGE>   67
 
                             MKS INSTRUMENTS, INC.
 
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
1.  DESCRIPTION OF BUSINESS:
 
     MKS Instruments, Inc. (the "Company") is a worldwide developer,
manufacturer, and supplier of instruments and components that are used to
measure, control and analyze gases in semiconductor manufacturing and similar
industrial manufacturing processes. The Company's products include pressure and
flow measurement and control instruments; vacuum gauges, valves and components;
and gas analysis instruments. The Company is subject to risks common to
companies in the semiconductor industry including, but not limited to, the
highly cyclical nature of the semiconductor industry leading to recurring
periods of over supply, development by the Company or its competitors of new
technological innovations, dependence on key personnel and the protection of
proprietary technology.
 
2.  SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES:
 
BASIS OF PRESENTATION
 
     The consolidated financial statements include the accounts of the Company
and its subsidiaries. All significant intercompany accounts and transactions
have been eliminated in consolidation. The Company has reflected the
approximately 77.5% owned foreign subsidiaries as wholly-owned subsidiaries
pursuant to common control accounting. Upon the closing of this offering for
which these financial statements are being prepared, the shares of the foreign
subsidiaries owned directly by the ultimate stockholders will be contributed to
the Company.
 
PRO FORMA BALANCE SHEET PRESENTATION (UNAUDITED)
 
     The Company intends to distribute the balance of its accumulated and
undistributed S corporation earnings from the proceeds of this offering for
which this registration statement is being prepared. The unaudited pro forma
balance sheet has been prepared assuming an estimated $35,926,000 distribution
was payable as of December 31, 1998. The remaining balance in retained earnings
represents accumulated earnings prior to the Company converting from a C
corporation to an S corporation in 1987, accumulated income in overseas
subsidiaries and differences between book and tax accumulated income.
 
HISTORICAL AND PRO FORMA (UNAUDITED) NET INCOME PER SHARE
 
     The Company computes basic and diluted earnings per share in accordance
with Statement of Financial Accounting Standards No. 128 ("SFAS 128") "Earnings
per Share." SFAS 128 requires both basic earnings per share, which is based on
the weighted average number of common shares outstanding, and diluted earnings
per share, which is based on the weighted average number of common shares
outstanding and all dilutive potential common equivalent shares outstanding. The
dilutive effect of options is determined under the treasury stock method using
the average market price for the period. Common equivalent shares are included
in the per share calculations where the effect of their inclusion would be
dilutive.
 
     Historical net income per share is not meaningful based upon the Company's
planned conversion from an S corporation to a C corporation upon the closing of
this offering for which these financial statements have been prepared.
Historical net income has been adjusted for the pro forma provision for income
taxes calculated assuming the Company was subject to income taxation as a C
corporation, at a pro forma tax rate of 38.0%. In accordance with a regulation
of the Securities and Exchange Commission, pro forma net income per share has
been presented for the year ended December 31, 1998 to reflect the effect of the
assumed issuance of 2,242,272 shares of common stock of the Company necessary to
be sold at the mid-point of the estimated initial public offering price in order
to fund the intended distribution of the accumulated and undistributed S
corporation earnings as of January 1, 1998.
 
                                       F-7
<PAGE>   68
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
     The following is a reconciliation of basic to diluted pro forma and
historical net income per share:
 
<TABLE>
<CAPTION>
                                                            FOR THE YEAR ENDED DECEMBER 31,
                                        ------------------------------------------------------------------------
                                                 1996                     1997                     1998
                                        ----------------------   ----------------------   ----------------------
                                        PRO FORMA   HISTORICAL   PRO FORMA   HISTORICAL   PRO FORMA   HISTORICAL
                                        ---------   ----------   ---------   ----------   ---------   ----------
<S>                                     <C>         <C>          <C>         <C>          <C>         <C>
Net income............................   $ 8,248     $12,503      $13,806     $20,290      $ 5,044     $ 7,186
Shares used in net income per common
  share -- basic......................    18,053      18,053       18,053      18,053       20,295      18,053
Effect of dilutive securities:
     Employee and director stock
       options........................        --          --          209         335          485         667
                                         -------     -------      -------     -------      -------     -------
Shares used in net income per common
  share -- diluted....................    18,053      18,053       18,262      18,388       20,780      18,720
                                         =======     =======      =======     =======      =======     =======
Net income per common share --
  basic...............................   $  0.46     $  0.69      $  0.76     $  1.12      $  0.25     $  0.40
                                         =======     =======      =======     =======      =======     =======
Net income per common share --
  diluted.............................   $  0.46     $  0.69      $  0.76     $  1.10      $  0.24     $  0.38
                                         =======     =======      =======     =======      =======     =======
</TABLE>
 
FOREIGN EXCHANGE
 
     The functional currency of the Company's foreign subsidiaries is the
applicable local currency. For those subsidiaries, assets and liabilities are
translated to U.S. dollars at year-end exchange rates. Income and expense
accounts are translated at the average exchange rates prevailing for the year.
The resulting translation adjustments are included in accumulated other
comprehensive income in consolidated stockholders' equity.
 
REVENUE RECOGNITION
 
     The Company recognizes revenue upon shipment. The Company accrues for
anticipated returns and warranty costs upon shipment.
 
CASH AND CASH EQUIVALENTS
 
     All highly liquid investments with an original maturity of three months or
less at the date of purchase are considered to be cash equivalents. Cash
equivalents consist of money market instruments.
 
INVESTMENTS
 
     The appropriate classification of investments in debt and equity securities
is determined at the time of purchase. Debt securities that the Company has both
the intent and ability to hold to maturity are carried at amortized cost. Debt
securities that the Company does not have the intent and ability to hold to
maturity or equity securities are classified either as "available-for-sale" or
as "trading" and are carried at fair value. Marketable equity securities are
carried at fair value and classified either as available-for-sale or trading.
Unrealized gains and losses on securities classified as available-for-sale are
included in accumulated other comprehensive income in consolidated stockholders'
equity. Unrealized gains and losses on securities classified as trading are
reported in earnings.
 
INVENTORIES
 
     Inventories are stated at the lower of cost or market. Cost is determined
on the first-in, first-out method.
 
                                       F-8
<PAGE>   69
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
PROPERTY, PLANT AND EQUIPMENT
 
     Property, plant and equipment are stated at cost. Equipment acquired under
capital leases is recorded at the present value of the minimum lease payments
required during the lease period. Expenditures for major renewals and
betterments that extend the useful lives of property, plant and equipment are
capitalized. Expenditures for maintenance and repairs are charged to expense as
incurred. When assets are sold or otherwise disposed of, the cost and related
accumulated depreciation are eliminated from the accounts and any resulting gain
or loss is recognized in earnings.
 
     Depreciation is provided on the straight-line method over the estimated
useful lives of 20 years for buildings and three to five years for machinery and
equipment. Leasehold improvements are amortized over the shorter of the lease
term or the estimated useful life of the lease.
 
RESEARCH AND DEVELOPMENT
 
     Research and development costs are expensed as incurred.
 
NEW ACCOUNTING PRONOUNCEMENTS
 
     In March 1998, the American Institute of Certified Public Accountants
issued Statement of Position (SOP) 98-1, "Accounting for the Costs of Software
Developed or Obtained for Internal Use" which provides guidance on the
accounting for the costs of software developed or obtained for internal use. SOP
98-1 is effective for fiscal years beginning after December 15, 1998. The
Company does not expect the SOP 98-1 to have a material impact on its financial
position or results of operations.
 
     In June 1998, the Financial Accounting Standards Board issued Statement of
Financial Accounting Standards (SFAS) No. 133, "Accounting for Derivative
Instruments and Hedging Activities." SFAS No. 133 requires that all derivative
instruments be recorded on the balance sheet at their fair value. Changes in the
fair value of derivatives are recorded each period in current earnings or other
comprehensive income, depending on whether a derivative is designated as part of
a hedge transaction and, if it is, the type of hedge transaction. The statement
is effective for all fiscal quarters of all fiscal years beginning after June
15, 1999. The Company has not yet determined the impact that the adoption SFAS
No. 133 will have on its financial position or results of operations.
 
USE OF ESTIMATES
 
     The preparation of financial statements in conformity with generally
accepted accounting principles requires management to make estimates and
assumptions that affect the reported amounts of assets and liabilities and
disclosure of contingent assets and liabilities at the dates of the financial
statements and the reported amounts of revenues and expenses during the
reporting periods. Actual results could differ from those estimates.
 
RECLASSIFICATION OF PRIOR YEAR BALANCES
 
     Certain reclassifications have been made to prior years' consolidated
financial statements to conform to the current presentation.
 
                                       F-9
<PAGE>   70
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
3.  FINANCIAL INSTRUMENTS AND RISK MANAGEMENT:
 
FOREIGN EXCHANGE RISK MANAGEMENT
 
     The Company uses forward exchange contracts and local currency purchased
options in an effort to reduce its exposure to currency fluctuations on future
U.S. dollar cash flows derived from foreign currency denominated sales
associated with the intercompany purchases of inventory. The Company has entered
into forward exchange contracts and local currency purchased options to hedge a
portion of its probable anticipated, but not firmly committed transactions. The
anticipated transactions whose risks are being hedged are the intercompany
purchases of inventory by the foreign subsidiaries from the U.S. parent for
resale in their local currency. The time period of the anticipated transactions
that are hedged generally approximate one year. The Company has also used
forward exchange contracts to hedge firm commitments. Market value gains and
losses on forward exchange contracts are recognized immediately in earnings
unless a firm commitment exists. Market value gains and premiums on local
currency purchased options on probable anticipated transactions and market value
gains and losses on forward exchange contracts hedging firm commitments are
recognized when the hedged transaction occurs. These contracts, which relate
primarily to Japanese and European currencies generally have terms of twelve
months or less. The Company does not hold or issue derivative financial
instruments for trading purposes.
 
     Realized and unrealized gains and losses on forward exchange contracts and
local currency purchased options that qualify for hedge accounting are
recognized in earnings in the same period as the underlying hedged item.
Realized and unrealized gains and losses on forward exchange contracts and local
currency purchased option contracts that do not qualify for hedge accounting are
recognized immediately in earnings. Forward exchange contracts receive hedge
accounting on firmly committed transactions when they are designated as a hedge
of the designated currency exposure and are effective in minimizing such
exposure. Options receive hedge accounting on probable anticipated transactions
when they are designated as a hedge of the currency exposure and are effective
in minimizing such exposure. The cash flows resulting from forward exchange
contracts and local currency purchased options that qualify for hedge accounting
are classified in the statement of cash flows as part of cash flows from
operating activities. Cash flows resulting from forward exchange contracts and
local currency purchased options that do not qualify for hedge accounting are
classified in the statement of cash flows as investing activities.
 
     Forward exchange contracts with notional amounts totaling none, $9,800,000,
and $8,000,000 to exchange foreign currencies for U.S. dollars, were outstanding
at December 31, 1996, 1997, and 1998, respectively. Of such forward exchange
contracts $6,900,000 and $7,800,000 to exchange Japanese yen for U.S. dollars,
were outstanding at December 31, 1997 and 1998, respectively. The forward
exchange contracts with notional amounts outstanding at December 31, 1998
totaling $8,000,000 do not qualify for hedge accounting and accordingly are
marked to market and recognized immediately in earnings. Local currency
purchased options with notional amounts totaling $3,722,000, $12,738,000, and
$10,221,000 to exchange foreign currencies for U.S. dollars were outstanding at
December 31, 1996, 1997, and 1998, respectively.
 
     Foreign exchange losses of $479,000, foreign exchange gains of $1,166,000
and foreign exchange losses of $168,000 on forward exchange contracts that did
not qualify for hedge accounting were recognized in earnings during 1996, 1997
and 1998, respectively, and are classified in Other income (expense), net. Gains
on forward exchange contracts that qualify for hedge accounting of $978,000 were
deferred and classified in other accrued expenses at December 31, 1996. Gains on
local currency purchased options deferred at December 31, 1996 that qualify for
hedge accounting of $200,000 were deferred in other accrued expenses. Gains on
forward exchange contracts and local currency purchased options that qualify
 
                                      F-10
<PAGE>   71
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
for hedge accounting are classified in cost of goods sold and totaled
$2,476,000, $1,178,000, and $310,000 for the years ended December 31, 1996,
1997, and 1998, respectively.
 
     The fair value of forward exchange contracts at December 31, 1998,
determined by applying period end currency exchange rates to the notional
contract amounts, amounted to a loss of $349,000. The fair values of local
currency purchased options at December 31, 1997 and 1998 which were obtained
through dealer quotes were immaterial.
 
     The Company recorded a foreign exchange translation loss on intercompany
payables of $1,000,000 and a foreign exchange translation gain on intercompany
payables of $1,000,000 in Other income (expense), net in 1997 and 1998,
respectively. Foreign exchange translation gains and losses from unhedged
intercompany balances were not material in 1996.
 
     The market risk exposure from forward exchange contracts is assessed in
light of the underlying currency exposures and is controlled by the initiation
of additional or offsetting foreign currency contracts. The market risk exposure
from options is limited to the cost of such investments. Credit risk exposure
from forward exchange contracts and local currency purchased options are
minimized as these instruments are contracted with a major financial
institution. The Company monitors the credit worthiness of this financial
institution and full performance is anticipated.
 
INTEREST RATE RISK MANAGEMENT
 
     The Company utilizes an interest rate swap to fix the interest rate on
certain variable rate term loans in order to minimize the effect of changes in
interest rates on earnings. In 1998, the Company entered into a four-year
interest rate swap agreement on a declining notional amount basis which
coincides with the scheduled principal payments with a major financial
institution for the notional amount of $10,528,000 equal to the term loans
described in Note 6. Under the agreement, the Company pays a fixed rate of 5.85%
on the notional amount and receives LIBOR. The interest differential payable or
accruable on the swap agreement is recognized on an accrual basis as an
adjustment to interest expense. The criteria used to apply hedge accounting for
this interest rate swap is based upon management designating the swap as a hedge
against the variable rate debt combined with the terms of the swap matching the
underlying debt including the notional amount, the timing of the interest reset
dates, the indices used and the paydates. At December 31, 1998, the fair value
of this interest rate swap, which represents the amount the Company would
receive or pay to terminate the agreement, is a net payable of $151,000, based
on dealer quotes. The variable rate received on the swap at December 31, 1998
was 5.5%.
 
     The market risk exposure from the interest rate swap is assessed in light
of the underlying interest rate exposures. Credit risk exposure from the swap is
minimized as the agreement is with a major financial institution. The Company
monitors the credit worthiness of this financial institution and full
performance is anticipated.
 
CONCENTRATIONS OF CREDIT RISK
 
     The Company's significant concentrations of credit risk consist principally
of cash and cash equivalents and trade accounts receivable. The Company
maintains cash and cash equivalents with financial institutions including the
bank it has borrowings with. Concentrations of credit risk with respect to trade
accounts receivable are limited due to the large number of geographically
dispersed customers. Credit is extended for all customers based on financial
condition and collateral is not required.
 
                                      F-11
<PAGE>   72
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
FAIR VALUE OF FINANCIAL INSTRUMENTS
 
     The fair value of the term loans, including the current portion,
approximates its carrying value given its variable rate interest provisions. The
fair value of mortgage notes is based on borrowing rates for similar instruments
and approximates its carrying value. For all other balance sheet financial
instruments, the carrying amount approximates fair value because of the short
period to maturity of these instruments.
 
4.  INVENTORIES:
 
     Inventories consist of the following:
 
<TABLE>
<CAPTION>
                                                                DECEMBER 31,
                                                             ------------------
                                                              1997       1998
                                                             -------    -------
<S>                                                          <C>        <C>
Raw material...............................................  $ 9,981    $ 7,544
Work in process............................................    7,241      5,718
Finished goods.............................................   12,741     11,202
                                                             -------    -------
                                                             $29,963    $24,464
                                                             =======    =======
</TABLE>
 
5.  PROPERTY, PLANT AND EQUIPMENT:
 
     Property, plant and equipment consist of the following:
 
<TABLE>
<CAPTION>
                                                                DECEMBER 31,
                                                             ------------------
                                                              1997       1998
                                                             -------    -------
<S>                                                          <C>        <C>
Land.......................................................  $ 8,350    $ 8,834
Buildings..................................................   26,241     26,020
Machinery and equipment....................................   24,861     27,394
Furniture and fixtures.....................................    9,697     10,578
Leasehold improvements.....................................      882      1,814
                                                             -------    -------
                                                              70,031     74,640
Less: accumulated depreciation and amortization............   36,055     41,915
                                                             -------    -------
                                                             $33,976    $32,725
                                                             =======    =======
</TABLE>
 
6.  DEBT:
 
CREDIT AGREEMENTS AND SHORT-TERM BORROWINGS
 
     In February 1996, the Company entered into loan agreements with two banks,
which provide access to a revolving credit facility. These agreements have since
been amended. The revolving credit facility, as amended, provides for
uncollateralized borrowings up to $30,000,000, which expires on December 31,
1999. Interest on borrowings is payable quarterly at either the banks' base rate
or the LIBOR Rate, as defined in the agreement, at the Company's option. At
December 31, 1997 and 1998, the Company had no borrowings under this revolving
credit facility.
 
     Additionally, certain of the Company's foreign subsidiaries have lines of
credit and short-term borrowing arrangements with various financial institutions
which provide for aggregate borrowings as of December 31, 1998 of up to
$15,003,000, which generally expire and are renewed at six month intervals. At
December 31, 1997 and 1998, total borrowings outstanding under these
arrangements were $10,721,000, and $9,687,000, respectively, at interest rates
ranging from 1.3% to 1.6%, and 1.3% to 1.7%, respectively. Foreign short-term
borrowings are generally collateralized by certain trade accounts receivable and
are guaranteed by a domestic bank.
 
                                      F-12
<PAGE>   73
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
LONG-TERM DEBT
 
     Long-term debt consists of the following:
 
<TABLE>
<CAPTION>
                                                                DECEMBER 31,
                                                              -----------------
                                                               1997      1998
                                                              -------   -------
<S>                                                           <C>       <C>
Term loans..................................................  $12,194   $10,528
Mortgage notes..............................................    3,624     3,572
                                                              -------   -------
Total long-term debt........................................   15,818    14,100
Less: current portion.......................................    2,070     2,058
                                                              -------   -------
Long-term debt less current portion.........................  $13,748   $12,042
                                                              =======   =======
</TABLE>
 
     On November 1, 1993, the Company entered into a term loan agreement with a
bank, which provided for borrowings of $10,000,000. Principal payments are
payable in equal monthly installments of $56,000 through October 1, 2000, with
the remaining principal payment due on November 1, 2000. The loan is
collateralized by certain land, buildings, and equipment. Interest is payable
monthly at either the bank's base rate, at a rate based on the long-term funds
rate, or at the LIBOR Rate, as defined in the agreement, at the Company's
option.
 
     On October 31, 1995, the Company also entered into a term loan agreement
with the same bank, which provided additional uncollateralized borrowings of
$7,000,000. Principal payments are payable in equal monthly installments of
$83,000 through June 1, 2002, with the remaining principal payment due on June
30, 2002. Interest is payable monthly at either the bank's base rate or at the
LIBOR Rate, as defined in the agreement, at the Company's option.
 
     At December 31, 1997 and 1998, the interest rates in effect for the term
loan borrowings were 6.975% and 7.131%, respectively.
 
     The terms of the revolving credit facility and term loan agreements, as
amended, contain, among other provisions, requirements for maintaining certain
levels of tangible net worth and other financial ratios. The agreement also
contains restrictions with respect to acquisitions. Under the most restrictive
covenant, the operating cash flow to debt service ratio for a fiscal quarter
shall not be less than 1.25 to 1.0. In the event of default of these covenants
or restrictions, any obligation then outstanding under the loan agreement shall
become payable upon demand by the bank. See Note 13 for subsequent event.
 
     The Company has loans outstanding from various foreign banks in the form of
mortgage notes at interest rates ranging from 2.0% to 6.2%. Principal and
interest are payable in monthly installments through 2010. The loans are
collateralized by mortgages on certain of the Company's foreign properties.
 
     Aggregate maturities of long-term debt over the next five years are as
follows:
 
<TABLE>
<CAPTION>
                                                                AGGREGATE
                  YEAR ENDING DECEMBER 31,                      MATURITIES
                  ------------------------                      ----------
<S>                                                             <C>
     1999...................................................     $ 2,058
     2000...................................................       7,343
     2001...................................................       1,405
     2002...................................................       1,329
     2003...................................................         422
     Thereafter.............................................       1,543
                                                                 -------
                                                                 $14,100
                                                                 =======
</TABLE>
 
                                      F-13
<PAGE>   74
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
7.  LEASE COMMITMENTS:
 
     The Company leases certain of its facilities and machinery and equipment
under capital and operating leases expiring in various years through 2002 and
thereafter. Generally, the facility leases require the Company to pay
maintenance, insurance and real estate taxes. Rental expense under operating
leases totaled $2,487,000, $2,478,000, and $2,388,000 for the years ended
December 31, 1996, 1997, and 1998, respectively.
 
     Minimum lease payments under operating and capital leases are as follows:
 
<TABLE>
<CAPTION>
                                                                                           CAPITAL
                                                                  OPERATING LEASES         LEASES
                                                              ------------------------    ---------
                  YEAR ENDING DECEMBER 31,                    REAL ESTATE    EQUIPMENT    EQUIPMENT
                  ------------------------                    -----------    ---------    ---------
<S>                                                           <C>            <C>          <C>
     1999...................................................    $1,484         $437        $1,202
     2000...................................................       882          251           974
     2001...................................................       660          130           537
     2002...................................................       153           36           333
     2003...................................................        84           13           116
     Thereafter.............................................        51           42            --
                                                                ------         ----        ------
Total minimum lease payments................................    $3,314         $909        $3,162
                                                                ======         ====        ======
Less: amounts representing interest.........................                                  344
                                                                                           ------
Present value of minimum lease payments.....................                                2,818
Less: current portion.......................................                                1,074
                                                                                           ------
Long-term portion...........................................                               $1,744
                                                                                           ======
</TABLE>
 
8.  STOCKHOLDERS' EQUITY:
 
COMMON STOCK
 
     The Company has two classes of common stock. Stockholders of Class A common
stock are entitled to voting rights with one vote for each share of common
stock. Stockholders of Class B common stock are not entitled to voting rights.
 
     Upon the closing of this offering for which this Registration Statement is
being prepared each outstanding share of Class A and Class B common stock of the
Company will be converted into an aggregate of 18,053,167 shares of common
stock.
 
STOCK OPTION PLANS
 
     On January 9, 1998, the stockholders of the Company approved the following:
(1) an increase in the number of shares that may be granted under the 1995 Stock
Incentive Plan to 3,750,000 shares of common stock; (2) the adoption of the 1997
Director Stock Option Plan pursuant to which options may be granted to purchase
up to an aggregate of 300,000 shares of common stock; (3) the adoption of the
1997 Employee Stock Purchase Plan pursuant to which the Company may issue up to
an aggregate of 450,000 shares of common stock; and (4) that 3,750,000 shares,
300,000 shares, and 450,000 shares of common stock be reserved for issuance
under the 1995 Stock Incentive Plan, the 1997 Director Stock Option Plan, and
the 1997 Employee Stock Purchase Plan, respectively.
 
     The Company grants options to employees under the 1995 Stock Incentive Plan
(the "Plan") and to directors under the 1996 Director Stock Option Plan (the
"Director Plan").
 
                                      F-14
<PAGE>   75
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
     At December 31, 1998 options to purchase 1,651,793 shares of the Company's
common stock were reserved for issuance under the Plan. At December 31, 1998,
under the Director Plan, options to purchase 28,932 shares of common stock were
reserved for issuance. Stock options are granted at 100% of the fair value of
the Company's common stock as determined by the Board of Directors on the date
of grant. In reaching the determination of fair value at the time of each grant,
the Board of Directors considered a range of factors, including the Company's
current financial position, its recent revenues, results of operations and cash
flows, its assessment of the Company's competitive position in its markets and
prospects for the future, the status of the Company's product development and
marketing efforts, current valuations for comparable companies and the
illiquidity of an investment in the Company's common stock. Generally, stock
options under the Plan vest 20% after one year and 5% per quarter thereafter,
and expire 10 years after the grant date. Under the Director Plan, the options
granted in 1996 vest over three years and options granted in 1997 and later vest
at the earlier of (1) the next annual meeting, (2) 13 months from date of grant
or (3) the effective date of an acquisition as defined in the Director Plan.
 
     The following table presents the activity for options under the Plan.
 
<TABLE>
<CAPTION>
                                           YEAR ENDED             YEAR ENDED             YEAR ENDED
                                        DECEMBER 31, 1996     DECEMBER 31, 1997       DECEMBER 31, 1998
                                       -------------------   --------------------   ---------------------
                                                  WEIGHTED               WEIGHTED                WEIGHTED
                                                  AVERAGE                AVERAGE                 AVERAGE
                                                  EXERCISE               EXERCISE                EXERCISE
                                       OPTIONS     PRICE      OPTIONS     PRICE      OPTIONS      PRICE
                                       --------   --------   ---------   --------   ----------   --------
<S>                                    <C>        <C>        <C>         <C>        <C>          <C>
Outstanding -- beginning of period...   608,270    $11.06      810,442    $4.43      1,564,449    $4.50
Granted..............................   810,442      4.43      785,657     4.57        629,969     6.80
Exercised............................        --        --           --       --             (2)    4.43
Forfeited or Expired.................  (608,270)    11.06      (31,650)    4.43        (96,209)    4.43
                                       --------    ------    ---------    -----     ----------    -----
Outstanding -- end of period.........   810,442    $ 4.43    1,564,449    $4.50      2,098,207    $5.20
Exercisable at end of period.........   114,782    $ 4.43      476,451    $4.43        778,473    $4.46
</TABLE>
 
     At December 31, 1998, Plan options included 1,436,588, 566,669, and 94,950
shares outstanding at exercise prices of $4.43, $6.67, and $8.00 per share. The
weighted average remaining contractual life of these options was 8.2 years.
 
     During 1996, 27,128 options were granted at an exercise price of $4.43 per
share under the Director Plan and were outstanding at December 31, 1996. Of
these options, 4,524 were exercisable at December 31, 1996. During 1997, options
for 3,620 shares were granted under the Director Plan at an exercise price of
$4.43 per share. Of these options, 30,748 were outstanding with 13,564
exercisable at the $4.43 per share price at December 31, 1997. During 1998,
options for 3,620 shares were granted under the Director Plan at an exercise
price of $8.00 per share. Of these options, 34,368 were outstanding with 26,228
exercisable at the $4.43 per share price at December 31, 1998.
 
     The Company has adopted the disclosure-only provisions of Statement of
Financial Accounting Standards No. 123 (SFAS No. 123), "Accounting for
Stock-Based Compensation." The Company has chosen to continue to account for
stock-based compensation using the intrinsic value method prescribed in
Accounting Principles Board Opinion No. 25, "Accounting for Stock Issued to
Employees" and related interpretations. Accordingly, compensation cost for stock
options is measured as the excess, if any, of the fair value of the Company's
stock at the date of grant over the amount an employee must pay to acquire the
stock.
 
     The disclosures required under SFAS No. 123 have been omitted as they are
not meaningful based upon the Company's planned conversion from an S corporation
to a C corporation upon the closing of this offering for which these financial
statements are being prepared. Had the fair value based method prescribed in
SFAS No. 123 been used to account for stock-based compensation cost, there would
have
 
                                      F-15
<PAGE>   76
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
been no change in pro forma net income and pro forma earnings per share from
that reported based on the following assumptions: dividend yield of 8%, risk
free interest rate of 5.44% and an expected life of 8 years.
 
9.  INCOME TAXES:
 
     The Company has elected to be taxed as an S corporation for federal and
certain states income tax purposes and, as a result, is not subject to Federal
taxation but is subject to state taxation on income in certain states. The
stockholders are liable for individual Federal and certain state income taxes on
their allocated portions of the Company's taxable income.
 
     The components of income before income taxes and the historical related
provision for income taxes consist of the following:
 
<TABLE>
<CAPTION>
                                                           YEAR ENDED DECEMBER 31,
                                                         ----------------------------
                                                          1996       1997       1998
                                                         -------    -------    ------
<S>                                                      <C>        <C>        <C>
Income before income taxes:
  United States........................................  $11,953    $21,858    $6,169
  Foreign..............................................    1,350        410     1,966
                                                         -------    -------    ------
                                                          13,303     22,268     8,135
Current taxes:
  State................................................      285      1,331       197
  Foreign..............................................      792        792       784
                                                         -------    -------    ------
                                                           1,077      2,123       981
                                                         -------    -------    ------
Deferred taxes:
  State................................................     (156)       (72)      (39)
  Foreign..............................................     (121)       (73)        7
                                                         -------    -------    ------
                                                            (277)      (145)      (32)
                                                         -------    -------    ------
Provision for income taxes.............................  $   800    $ 1,978    $  949
                                                         =======    =======    ======
</TABLE>
 
     As the Company is not subject to Federal income taxes, a reconciliation of
the effective tax rate to the Federal statutory rate is not meaningful.
 
     At December 31, 1996, 1997, and 1998 the components of the deferred tax
asset and deferred tax liability were as follows:
 
<TABLE>
<CAPTION>
                                                                 DECEMBER 31,
                                                           ------------------------
                                                           1996      1997      1998
                                                           ----      ----      ----
<S>                                                        <C>       <C>       <C>
Deferred tax assets (liabilities):
  Inventories............................................  $234      $344      $265
  Intercompany profits...................................   160       214       152
  Compensation...........................................    72        77       127
  Investment booked under the equity method..............   (28)      (41)      (59)
  Other..................................................   (34)      (45)       96
                                                           ----      ----      ----
          Total..........................................  $404      $549      $581
                                                           ====      ====      ====
</TABLE>
 
                                      F-16
<PAGE>   77
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
10.  EMPLOYEE BENEFIT PLANS:
 
     The Company has a 401(k) profit-sharing plan for U.S. employees meeting
certain requirements in which eligible employees may contribute from 1% up to
12% of their compensation. The Company, at its discretion, may provide a
matching contribution which will generally match up to the first 2% of each
participant's compensation, plus 25% of the next 4% of compensation. At the
discretion of the Board of Directors, the Company may also make additional
contributions for the benefit of all eligible employees. The Company's
contributions are generally paid annually, and were $2,170,000 and $2,500,000
for the years ended December 31, 1996 and 1997. Approximately $1,400,000 has
been accrued as the estimated Company contribution for the year ended December
31, 1998 and is included in accrued compensation.
 
     The Company maintains a bonus plan which provides cash awards to key
employees, at the discretion of the Compensation Committee of the Board of
Directors, based upon operating results and employee performance. Bonus expense
to key employees was none, $1,425,000, and none for the years ended December 31,
1996, 1997, and 1998, respectively.
 
11.  RESTRUCTURING:
 
     In 1996, the Company recorded a restructuring charge of $1,400,000,
primarily related to reduction of personnel and the closure of facilities in
Phoenix, AZ and San Jose, CA. These charges include $425,000 of severance pay,
$710,000 of lease commitments, and $265,000 for the write-off of leasehold
improvements. The facilities closure concluded during 1997. The remaining
balance of approximately $126,000 for lease commitments is included in Other
accrued expenses in the accompanying balance sheet at December 31, 1998.
 
12.  GEOGRAPHIC FINANCIAL INFORMATION AND SIGNIFICANT CUSTOMER:
 
     See Note 1 for a brief description of the Company's business. The Company
is organized around two similar product lines domestically and by geographic
locations internationally and has three reportable segments: North America, Far
East, and Europe. Net sales to unaffiliated customers are based on the location
in which the sale originated. Transfers between geographic areas are at
negotiated transfer prices and have been eliminated from consolidated net sales.
Income from operations consists of total net sales less operating expenses and
does not include either interest income, interest expense or income taxes. The
Company had one customer comprising 15%, 22% and 16% of net sales for the years
ended December 31, 1996, 1997, and 1998, respectively. This data is presented in
accordance with SFAS 131, "Disclosures About Segments of an Enterprise and
Related Information," which the Company has retroactively adopted for all
periods presented.
 
                                      F-17
<PAGE>   78
                             MKS INSTRUMENTS, INC.
 
           NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)
                  (TABLES IN THOUSANDS, EXCEPT PER SHARE DATA)
 
<TABLE>
<CAPTION>
                                                  YEAR ENDED DECEMBER 31, 1998
                                        ------------------------------------------------
                                        NORTH AMERICA    FAR EAST    EUROPE      TOTAL
                                        -------------    --------    -------    --------
<S>                                     <C>              <C>         <C>        <C>
Net sales to unaffiliated customers...    $ 95,607       $23,902     $20,254    $139,763
Intersegment net sales................      26,657           290       1,015      27,962
Depreciation and amortization.........       5,627           210         405       6,242
Income from operations................       6,319         1,298       1,518       9,135
Segment assets........................      65,560        20,768       9,904      96,232
Long-lived assets.....................      28,960         5,655       3,084      37,699
Capital expenditures..................       2,635           179         323       3,137
                                                  YEAR ENDED DECEMBER 31, 1997
                                         -----------------------------------------------
 
Net sales to unaffiliated customers...    $138,186       $31,559     $18,335    $188,080
Intersegment net sales................      35,429           225         749      36,403
Depreciation and amortization.........       5,096           259         357       5,712
Income from operations................      22,847           886         230      23,963
Segment assets........................      77,302        19,906       9,328     106,536
Long-lived assets.....................      30,738         4,904       3,015      38,657
Capital expenditures..................       2,899           128         242       3,269
                                                  YEAR ENDED DECEMBER 31, 1996
                                         -----------------------------------------------
 
Net sales to unaffiliated customers...    $121,061       $31,066     $18,735    $170,862
Intersegment net sales................      34,100           199       1,426      35,725
Depreciation and amortization.........       5,145           388         387       5,920
Income from operations................      14,534           653         881      16,068
Segment assets........................      66,593        18,524       9,883      95,000
Long-lived assets.....................      33,402         5,554       3,551      42,507
Capital expenditures..................       8,332           208         877       9,417
</TABLE>
 
     Included in North America are the United States and Canada. Net sales to
unaffiliated customers from the United States were $119,423,000, $136,653,000
and $94,449,000 for the years ended December 31, 1996, 1997 and 1998,
respectively. Long-lived assets within the United States amounted to
$33,315,000, $30,667,000 and $28,902,000 at December 31, 1996, 1997, and 1998,
respectively.
 
     Included in the Far East are Japan, Korea and Singapore. Included in Europe
are Germany, France and the United Kingdom. Net sales to unaffiliated customers
from Japan were $28,242,000, $28,184,000 and $21,153,000 for the years ended
December 31, 1996, 1997 and 1998, respectively. Long-lived assets within Japan
amounted to $5,141,000, $4,792,000 and $5,431,000 at December 31, 1996, 1997 and
1998, respectively.
 
13.  SUBSEQUENT EVENTS:
 
     On January 28, 1999, the Company amended its revolving credit facility and
its term loan agreements described in Note 6. The amendments include revised
quarterly cash flow to debt service ratios. The most restrictive covenant is the
cash flow to debt service ratio of 1.25 to 1.0 in the fourth quarter of 1999 and
thereafter.
 
     On February 24, 1999 the Company effected a 3-for-2 stock split, in the
form of a stock dividend of its common stock and increased the number of
authorized shares of common stock to 30,000,000. Accordingly, all share data has
been restated to reflect the common stock split.
 
                                      F-18
<PAGE>   79
 
                      [This Page Intentionally Left Blank]
<PAGE>   80
 
                      [This Page Intentionally Left Blank]
<PAGE>   81
INSIDE BACK COVER (PG.5):

The inside back cover graphically depicts MKS's message of being a worldwide
provider of process control solutions. It is produced in four-color process. In
the center of the page is a photo of the Earth, with the tag line "Providing
Solutions Around the Process, Around the World" wrapping around the photo. The
word "Solutions" is highlighted with slightly larger type size. The background
of the page is dark, with the MKS logo appearing at the top right, knocking out
to white. Photos of MKS's products surround the photo of the Earth and include
MKS Baratron Capacitance Manometers, a Throttling Poppet Valve, a Pressure
Controller, Mass Flow Controllers, an In-Situ Flow Verifier, a Direct Liquid
Injection Subsystem and a Residual Gas Analyzer.


<PAGE>   82
 
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
 
                                6,500,000 SHARES
 
                                      LOGO
 
                                  COMMON STOCK
 
                            ------------------------
                                   Prospectus
                                          , 1999
                            ------------------------
 
                     NationsBanc Montgomery Securities LLC
 
                          Donaldson, Lufkin & Jenrette
 
                                Lehman Brothers
 
     Until             , 1999 (25 days after the date of this prospectus), all
dealers effecting transactions in the common stock, whether or not participating
in this distribution, may be required to deliver a prospectus. This is in
addition to the obligation of dealers to deliver a prospectus when acting as
underwriters and with respect to their unsold allotments or subscriptions.
 
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>   83
 
                                    PART II
 
                     INFORMATION NOT REQUIRED IN PROSPECTUS
 
ITEM 13.  OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.
 
     Estimated expenses payable in connection with the sale of the common stock
offered hereby are as follows:
 
<TABLE>
<S>                                                           <C>
SEC Registration Fee........................................  $ 35,327
NASD Filing Fee.............................................  $ 12,708
Printing, Engraving and Mailing Expenses....................  $120,000
Nasdaq Listing Fee..........................................  $ 95,000
Legal Fees and Expenses.....................................  $150,000
Accounting Fees and Expenses................................  $150,000
Blue Sky Fees and Expenses..................................  $  7,500
Transfer Agent and Registrar Fees...........................  $ 10,000
Miscellaneous...............................................  $ 19,465
                                                              --------
          Total.............................................  $600,000
                                                              ========
</TABLE>
 
- ---------------
 
     The Company will bear all expenses shown above.
 
ITEM 14.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.
 
     Section 67 of Chapter 156B of the Massachusetts General Laws provides that
a corporation may indemnify its directors and officers to the extent specified
in or authorized by (1) the articles of organization; (2) a by-law adopted by
the stockholders; or (3) a vote adopted by the holders of a majority of the
shares of stock entitled to vote on the election of directors. In all instances,
the extent to which a corporation provides indemnification to its directors and
officers under Section 67 is optional. In its Amended and Restated Articles of
Organization (the "Articles of Organization"), the Registrant has elected to
commit to provide indemnification to its directors and officers in specified
circumstances. Generally, Article 6 of the Registrant's Articles of Organization
provides that the Registrant shall indemnify directors and officers of the
Registrant against liabilities and expenses arising out of legal proceedings
brought against them by reason of their status as directors or officers or by
reason of their agreeing to serve, at the request of the Registrant, as a
director or officer with another organization. Under this provision, a director
or officer of the Registrant shall be indemnified by the Registrant for all
costs and expenses (including attorneys' fees), judgments, liabilities and
amounts paid in settlement of such proceedings, even if he is not successful on
the merits, if he acted in good faith in the reasonable belief that his action
was in the best interests of the Registrant. The Board of Directors may
authorize advancing litigation expenses to a director or officer at his request
upon receipt of an undertaking by any such director of officer to repay such
expenses if it is ultimately determined that he is not entitled to
indemnification for such expenses.
 
     Article 6 of the Registrant's Articles of Organization eliminates the
personal liability of the Registrant's directors to the Registrant or its
stockholders for monetary damages for breach of a director's fiduciary duty,
except to the extent Chapter 156B of the Massachusetts General Laws prohibits
the elimination or limitation of such liability.
 
     The Underwriting Agreement, a form of which is filed at Exhibit 1.1 to this
Registration Statement on Form S-1 (the "Underwriting Agreement"), provides that
the underwriters are obligated under certain circumstances to indemnify
directors, officers and controlling persons of the Registrant against certain
liabilities, including liabilities under the Securities Act of 1933, as amended
(the "Securities Act"). Reference is made to the form of Underwriting Agreement.
 
                                      II-1
<PAGE>   84
 
     The Company has obtained directors and officers liability insurance for the
benefit of its directors and certain of its officers.
 
ITEM 15.  RECENT SALES OF UNREGISTERED SECURITIES.
 
     In the three years preceding the filing of this Registration Statement, the
Registrant sold 2 shares of its common stock for total proceeds of $6.64 to an
employee. The registrant awarded options to purchase 837,570 shares of common
stock at a weighted average exercise price of $4.43 per share and 789,277 shares
of common stock at a weighted average exercise price of $4.57 per share, in 1996
and 1997, respectively, to employees and directors of the Company.
 
     In 1998, the registrant awarded options to purchase shares of common stock
to employees and directors of the Company on the dates, in the amounts, and at
the exercise price set forth below:
 
<TABLE>
<CAPTION>
                                                             NUMBER OF        EXERCISE PRICE
DATE                                                          OPTIONS           PER SHARE
- ----                                                     -----------------    --------------
<S>                                                      <C>                  <C>
January 9, 1998........................................         3,620             $8.00
January 26, 1998.......................................        31,650             $8.00
March 31, 1998.........................................        31,650             $8.00
July 9, 1998...........................................       450,000             $6.67
November 10, 1998......................................       116,669             $6.67
</TABLE>
 
     The grant of options were exempt from registration under the Securities Act
by virtue of the provisions of Section 4(2) of the Securities Act or Rule 701
thereunder.
 
ITEM 16.  EXHIBITS AND FINANCIAL STATEMENT SCHEDULES.
 
     (a) Exhibits:
 
   
<TABLE>
<CAPTION>
  EX. NO.                           DESCRIPTION
  -------                           -----------
  <C>       <S>
     1.1    Form of Underwriting Agreement
    +3.1    Restated Articles of Organization, as amended
     3.2    Form of Amended and Restated Articles of Organization
    +3.3    By-Laws, as amended
    +3.4    Form of Amended and Restated By-Laws
    +4.1    Specimen certificate representing the common stock
    +5.1    Opinion of Hale and Dorr LLP
   +10.1    Amended and Restated 1995 Stock Incentive Plan
   +10.2    1996 Amended and Restated 1996 Director Stock Option Plan
   +10.3    1997 Director Stock Option Plan
   +10.4    1999 Employee Stock Purchase Plan
   +10.5    Amended and Restated Employment Agreement dated as of
            December 15, 1995 between Leo Berlinghieri and the
            Registrant
   +10.6    Amended and Restated Employment Agreement dated as of
            December 15, 1995 between John J. Sullivan and the
            Registrant
   +10.7    Amended and Restated Employment Agreement dated as of
            December 15, 1995 between Ronald C. Weigner and the
            Registrant
   +10.8    Amended and Restated Employment Agreement dated as of
            December 15, 1995 between William D. Stewart and the
            Registrant
   +10.9    Loan Agreement dated as of October 31, 1995, as last amended
            January 28, 1999, by and between the First National Bank of
            Boston and the Registrant
  +10.10    Lease Agreement dated as of October 12, 1989, as extended
            November 1, 1998, by and between Aspen Industrial Park
            Partnership and the Registrant
  +10.11    Loan Agreement dated as of November 1, 1993, as last amended
            January 28, 1999, between the First National Bank of Boston
            and the Registrant
</TABLE>
    
 
                                      II-2
<PAGE>   85
 
   
<TABLE>
<CAPTION>
  EX. NO.                           DESCRIPTION
  -------                           -----------
  <C>       <S>
   +10.12   Lease dated as of September 21, 1995 by and between General
            American Life Insurance Company and the Registrant
   +10.13   Loan Agreement dated as of February 23, 1996, as last
            amended January 28, 1999, between BankBoston, N.A., Chemical
            Bank and the Registrant
   +10.14   Revolving Credit Note ($8,000,000) dated February 23, 1996
            between Chemical Bank, The First National Bank of Boston and
            the Registrant
   +10.15   Revolving Credit Note ($12,000,000) dated February 23, 1996
            between Chemical Bank, The First National Bank of Boston and
            the Registrant
   +10.16   Promissory Note dated as of August 1990 between Jefferson
            National Life Insurance Company and the Registrant
 +**10.17   Comprehensive Supplier Agreement #982812 dated October 23,
            1998 by and between Applied Materials, Inc. and the
            Registrant
 +**10.18   Management Incentive Program
   +10.19   Lease dated as of December 21, 1989, as last amended
            December 1996, between Walpole Park South II Trust and the
            Registrant
   +10.20   Lease dated as of January 1, 1996 between MiFuji Kanzai Co.
            Ltd. and the Registrant (covering Floor 5)
   +10.21   Lease dated as of April 21, 1997 between MiFuji Kanzai Co.
            Ltd. and the Registrant (covering Floors 1 and 2)
   +10.22   Split-Dollar Agreement dated as of September 12, 1991
            between the Registrant, John R. Bertucci and Claire R.
            Bertucci and Richard S. Chute, Trustees of the John R.
            Bertucci Insurance Trust of January 10, 1986
   +10.23   Split-Dollar Agreement dated as of September 12, 1991
            between the Registrant, John R. Bertucci and John R.
            Bertucci and Thomas H. Belknap, Trustees of the Claire R.
            Bertucci Insurance Trust of January 10, 1986
    10.24   Form of Tax Indemnification and S Corporation Distribution
            Agreement
   +10.25   Employment Agreement dated March 7, 1997 between Joseph
            Maher and the Registrant
    10.26   Form of Contribution Agreement
   +21.1    Subsidiaries of the Registrant
   +23.1    Consent of Hale and Dorr LLP (contained in Exhibit 5.1)
    23.2    Consent of PricewaterhouseCoopers LLP
   +24      Power of Attorney (included on Page II-5)
   +27      Financial Data Schedule
</TABLE>
    
 
- ---------------
   
** Confidential materials omitted and filed separately with the Securities and
Exchange Commission.
    
 + Previously filed.
 
     (b) FINANCIAL STATEMENTS SCHEDULES
 
     Report of Independent Accountants on Schedule II -- Valuation and
Qualifying Accounts
 
     Schedule II -- Valuation and Qualifying Accounts
 
ITEM 17.  UNDERTAKINGS.
 
     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
registrant pursuant to the foregoing provisions, or otherwise, the registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as expressed in the Act and is,
therefore, unenforceable. In the event that a claim for indemnification against
such liabilities (other than the payment by the registrant of expenses incurred
or paid by a director, officer or controlling person of the registrant in the
successful defense of any action, suit or proceeding) is asserted by such
director, officer and controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the questions whether such indemnification by it is against public
policy as expressed in the Act and will be governed by the final adjudication of
such issue.
 
                                      II-3
<PAGE>   86
 
     The undersigned registrant hereby undertakes that:
 
     (1) For purposes of determining any liability under the Securities Act of
1933, the information omitted from the form of prospectus filed as part of a
registration statement in reliance upon Rule 430A and contained in the form of
prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or 497(h)
under the Securities Act shall be deemed to be part of this registration
statement as of the time it was declared effective.
 
     (2) For the purpose of determining any liability under the Securities Act
of 1933, each post-effective amendment that contains a form of prospectus shall
be deemed to be a new registration statement relating to the securities offer
therein, and this offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.
 
     The undersigned registrant hereby further undertakes to provide to the
underwriters at the closing specified in the underwriting agreements,
certificates in such denominations and registered in such names as required by
the underwriters to permit prompt delivery to each purchaser.
 
                                      II-4
<PAGE>   87
 
                                   SIGNATURES
 
   
     Pursuant to the requirements of the Securities Act of 1933, the Registrant
has duly caused this amendment to the Registration Statement (File No.
333-71363) to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Andover, Commonwealth of Massachusetts, on this 23rd
day of March, 1999.
    
 
                                          MKS INSTRUMENTS, INC.
 
   
                                          By:     /s/ JOHN R. BERTUCCI
    
                                            ------------------------------------
   
                                                      JOHN R. BERTUCCI
    
   
                                              CHAIRMAN OF THE BOARD, PRESIDENT
                                                AND CHIEF EXECUTIVE OFFICER
    
 
     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.
 
   
<TABLE>
<CAPTION>
                    SIGNATURES                                      TITLE                     DATE
                    ----------                                      -----                     ----
<C>                                                   <S>                                <C>
 
               /s/ JOHN R. BERTUCCI*                  Chairman of the Board of           March 23, 1999
- ---------------------------------------------------   Directors, President and Chief
                 JOHN R. BERTUCCI                     Executive Officer (Principal
                                                      Executive Officer)
 
              /s/ RONALD C. WEIGNER*                  Vice President and Chief           March 23, 1999
- ---------------------------------------------------   Financial Officer (Principal
                 RONALD C. WEIGNER                    Financial and Accounting Officer)
 
               /s/ RICHARD S. CHUTE*                  Director                           March 23, 1999
- ---------------------------------------------------
                 RICHARD S. CHUTE
 
               /s/ OWEN W. ROBBINS*                   Director                           March 23, 1999
- ---------------------------------------------------
                  OWEN W. ROBBINS
 
              /s/ ROBERT J. THERRIEN*                 Director                           March 23, 1999
- ---------------------------------------------------
                ROBERT J. THERRIEN
 
               /s/ LOUIS P. VALENTE*                  Director                           March 23, 1999
- ---------------------------------------------------
                 LOUIS P. VALENTE
 
*By: /s/ JOHN R. BERTUCCI
- --------------------------------------------------
     JOHN R. BERTUCCI
     ATTORNEY-IN-FACT
</TABLE>
    
 
                                      II-5
<PAGE>   88
 
                       REPORT OF INDEPENDENT ACCOUNTANTS
 
To the Board of Directors and Stockholders
  of MKS Instruments, Inc.:
 
Our audits of the consolidated financial statements referred to in our report
dated January 22, 1999, except for the information in the first and second
paragraph of Note 13 as to which the date is January 28, 1999 and February 24,
1999, respectively, of MKS Instruments, Inc. also included an audit of the
consolidated financial statement schedule listed in Item 16(b) herein. In our
opinion, this consolidated financial statement schedule presents fairly, in all
material respects, the information set forth therein when read in conjunction
with the related consolidated financial statements.
 
                                            PRICEWATERHOUSECOOPERS LLP
 
Boston, Massachusetts
January 22, 1999
 
                                       S-1
<PAGE>   89
 
                                                                     SCHEDULE II
 
                             MKS INSTRUMENTS, INC.
 
                       VALUATION AND QUALIFYING ACCOUNTS
                                 (IN THOUSANDS)
 
<TABLE>
<CAPTION>
                                                                                                BALANCE
                                                        BALANCE AT   PROVISION                  AT END
                                                        BEGINNING    CHARGED TO    ACCOUNTS       OF
                                                        OF PERIOD     EXPENSE     WRITTEN OFF   PERIOD
                                                        ----------   ----------   -----------   -------
<S>                                                     <C>          <C>          <C>           <C>
YEAR ENDED DECEMBER 31, 1996
  Allowance for Doubtful Accounts.....................     $542          (20)          40        $482
YEAR ENDED DECEMBER 31, 1997
  Allowance for Doubtful Accounts.....................     $482          258          130        $610
YEAR ENDED DECEMBER 31, 1998
  Allowance for Doubtful Accounts.....................     $610          253          207        $656
</TABLE>
 
                                       S-2
<PAGE>   90
 
                                 EXHIBIT INDEX
 
   
<TABLE>
<CAPTION>
                                                                          SEQUENTIALLY
                                                                            NUMBERED
EX. NO.                           DESCRIPTION                                 PAGE
- -------                           -----------                             ------------
<C>       <S>                                                             <C>
    1.1   Form of Underwriting Agreement
   +3.1   Restated Articles of Organization, as amended
    3.2   Form of Amended and Restated Articles of Organization
   +3.3   By-Laws, as amended
   +3.4   Form of Amended and Restated By-Laws
   +4.1   Specimen certificate representing the common stock
   +5.1   Opinion of Hale and Dorr LLP
  +10.1   Amended and Restated 1995 Stock Incentive Plan
  +10.2   1996 Amended and Restated 1996 Director Stock Option Plan
  +10.3   1997 Director Stock Option Plan
  +10.4  1999 Employee Stock Purchase Plan
  +10.5   Amended and Restated Employment Agreement dated as of
          December 15, 1995 between Leo Berlinghieri and the
          Registrant
  +10.6   Amended and Restated Employment Agreement dated as of
          December 15, 1995 between John J. Sullivan and the
          Registrant
  +10.7   Amended and Restated Employment Agreement dated as of
          December 15, 1995 between Ronald C. Weigner and the
          Registrant
  +10.8   Amended and Restated Employment Agreement dated as of
          December 15, 1995 between William D. Stewart and the
          Registrant
  +10.9   Loan Agreement dated as of October 31, 1995, as last amended
          January 28, 1999, by and between the First National Bank of
          Boston and the Registrant
  +10.10  Lease Agreement dated as of October 12, 1989, as extended
          November 1, 1998, by and between Aspen Industrial Park
          Partnership and the Registrant
  +10.11  Loan Agreement dated as of November 1, 1993, as last amended
          January 28, 1999, between the First National Bank of Boston
          and the Registrant
  +10.12  Lease dated as of September 21, 1995 by and between General
          American Life Insurance Company and the Registrant
  +10.13  Loan Agreement dated as of February 23, 1996, as last
          amended January 28, 1999, between BankBoston, N.A., Chemical
          Bank and the Registrant
  +10.14  Revolving Credit Note ($8,000,000) dated February 23, 1996
          between Chemical Bank, The First National Bank of Boston and
          the Registrant
  +10.15  Revolving Credit Note ($12,000,000) dated February 23, 1996
          between Chemical Bank, The First National Bank of Boston and
          the Registrant
  +10.16  Promissory Note dated as of August 1990 between Jefferson
          National Life Insurance Company and the Registrant
+**10.17  Comprehensive Supplier Agreement #982812 dated October 23,
          1998 by and between Applied Materials, Inc. and the
          Registrant
+**10.18  Management Incentive Program
  +10.19  Lease dated as of December 21, 1989, as last amended
          December 1996, between Walpole Park South II Trust and the
          Registrant
</TABLE>
    
<PAGE>   91
 
   
<TABLE>
<CAPTION>
                                                                          SEQUENTIALLY
                                                                            NUMBERED
EX. NO.                           DESCRIPTION                                 PAGE
- -------                           -----------                             ------------
<C>       <S>                                                             <C>
 +10.20   Lease dated as of January 1, 1996 between MiFuji Kanzai Co.
          Ltd. and the Registrant (covering Floor 5)
 +10.21   Lease dated as of April 21, 1997 between MiFuji Kanzai Co.
          Ltd. and the Registrant (covering Floors 1 and 2) 
 +10.22   Split-Dollar Agreement dated as of September 12, 1991
          between the Registrant, John R. Bertucci and Claire R.
          Bertucci and Richard S. Chute, Trustees of the John R.
          Bertucci Insurance Trust of January 10, 1986
 +10.23   Split-Dollar Agreement dated as of September 12, 1991
          between the Registrant, John R. Bertucci and John R.
          Bertucci and Thomas H. Belknap, Trustees of the Claire R.
          Bertucci Insurance Trust of January 10, 1986
  10.24   Form of Tax Indemnification and S Corporation Distribution
          Agreement
 +10.25   Employment Agreement dated March 7, 1997 between Joseph
          Maher and the Registrant
  10.26   Form of Contribution Agreement
 +21.1    Subsidiaries of the Registrant
 +23.1    Consent of Hale and Dorr LLP (contained in Exhibit 5.1)
  23.2    Consent of PricewaterhouseCoopers LLP
 +24      Power of Attorney (included on Page II-5)
 +27      Financial Data Schedule
</TABLE>
    
 
- ---------------
   
** Confidential materials omitted and filed separately with the Securities and
Exchange Commission.
    
 + Previously filed.

<PAGE>   1
                                                                     Exhibit 1.1



   
                      NATIONSBANC MONTGOMERY SECURITIES LLC
                           FORM UNDERWRITING AGREEMENT
                                 Execution Copy
    



                                6,500,000 Shares



                              MKS Instruments, Inc.



                                  Common Stock



                             Underwriting Agreement

                              dated March __, 1999



<PAGE>   2


                                6,500,000 Shares

                              MKS INSTRUMENTS, INC.

                                  Common Stock

                             UNDERWRITING AGREEMENT


                                                               March ___, 1999

NATIONSBANC MONTGOMERY SECURITIES LLC
DONALDSON, LUFKIN & JENRETTE SECURITIES CORPORATION
LEHMAN BROTHERS INC.
As Representatives of the several Underwriters
c/o NATIONSBANC MONTGOMERY SECURITIES LLC
600 Montgomery Street
San Francisco, California  94111

Ladies and Gentlemen:

     Introductory. MKS Instruments, Inc., a Massachusetts corporation (the
"Company), proposes to issue and sell to the several underwriters named in
Schedule A (the "Underwriters") an aggregate of 6,000,000 shares of its Common
Stock, no par value per share (the "Common Stock") and the stockholders of the
Company named in Schedule B hereto (the "Selling Stockholders") severally
propose to sell to the Underwriters an aggregate of 500,000 shares of Common
Stock, each Selling Stockholder selling the amount set forth opposite such
Selling Stockholder's name in Schedule B (the aggregate of 6,500,000 shares to
be sold by the Company and the Selling Stockholders is herein called the "Firm
Common Shares"). In addition, the Company has granted to the Underwriters an
option to purchase up to an additional 975,000 shares of Common Stock (the
aggregate of additional shares to be sold by the Company is herein called the
"Optional Common Shares"), as provided in Section 2. The Firm Common Shares and,
if and to the extent such option is exercised, the Optional Common Shares are
collectively called the "Common Shares". NationsBanc Montgomery Securities LLC
("NMS"), Donaldson, Lufkin & Jenrette Securities Corporation and Lehman
Brothers, Inc. have agreed to act as representatives of the several Underwriters
(in such capacity, the "Representatives") in connection with the offering and
sale of the Common Shares.

     The Company has prepared and filed with the Securities and Exchange
Commission (the "Commission") a registration statement on Form S-1 (File No.
333-71363), which contains a form of prospectus to be used in connection with
the public offering and sale of the Common Shares. Such registration statement,
as amended, including the financial statements, exhibits and schedules thereto,
in the form in which it was declared effective by the Commission under the
Securities Act of 1933 and the rules and regulations promulgated thereunder
(collectively, the "Securities Act"), including any information deemed to be a
part thereof at the time of effectiveness pursuant to Rule 430A or Rule 434
under the Securities Act, is called the "Registration Statement". Any
registration statement filed by the Company pursuant to Rule 462(b) under the
Securities Act is called the "Rule 462(b) Registration


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<PAGE>   3


   
Statement", and from and after the date and time of filing of the Rule 462(b)
Registration Statement the term "Registration Statement" shall include the Rule
462(b) Registration Statement. Such prospectus, in the form first used by the
Underwriters to confirm sales of the Common Shares, is called the "Prospectus";
provided, however, if the Company has, with the consent of NMS, elected to rely
upon Rule 434 under the Securities Act, the term "Prospectus" shall mean the
Company's prospectus subject to completion (each, a "preliminary prospectus")
dated March 2, 1999 (such preliminary prospectus is called the "Rule 434
preliminary prospectus"), together with the applicable term sheet (the "Term
Sheet") prepared and filed by the Company with the Commission under Rules 434
and 424(b) under the Securities Act and all references in this Agreement to the
date of the Prospectus shall mean the date of the Term Sheet. All references in
this Agreement to the Registration Statement, the Rule 462(b) Registration
Statement, a preliminary prospectus, the Prospectus or the Term Sheet, or any
amendments or supplements to any of the foregoing, shall include any copy
thereof filed with the Commission pursuant to its Electronic Data Gathering,
Analysis and Retrieval System ("EDGAR").
    

     Each of the Company and the Selling Stockholders hereby confirms its
agreements with the Underwriters as follows:

Section 1A. Representations and Warranties of the Company. The Company hereby
represents, warrants and covenants to each Underwriter as follows:

     (a) Compliance with Registration Requirements. The Registration Statement
and any Rule 462(b) Registration Statement have been declared effective by the
Commission under the Securities Act. The Company has complied to the
Commission's satisfaction with all requests of the Commission for additional or
supplemental information. No stop order suspending the effectiveness of the
Registration Statement or any Rule 462(b) Registration Statement is in effect
and no proceedings for such purpose have been instituted or are pending or, to
the best knowledge of the Company, are contemplated or threatened by the
Commission.

Each preliminary prospectus and the Prospectus when filed complied in all
material respects with the Securities Act and, if filed by electronic
transmission pursuant to EDGAR (except as may be permitted by Regulation S-T
under the Securities Act), was textually identical to the copy thereof delivered
to the Underwriters for use in connection with the offer and sale of the Common
Shares. Each of the Registration Statement, any Rule 462(b) Registration
Statement and any post-effective amendment thereto, at the time it became
effective and at all subsequent times up to and including the Closing Date,
complied and will comply in all material respects with the Securities Act and
did not and will not contain any untrue statement of a material fact or omit to
state a material fact required to be stated therein or necessary to make the
statements therein not misleading. The Prospectus, as amended or supplemented,
as of its date and at all subsequent times up to and including the Closing Date,
did not and will not contain any untrue statement of a material fact or omit to
state a material fact necessary in order to make the statements therein, in the
light of the circumstances under which they were made, not misleading. The
representations and warranties set forth in the two immediately preceding
sentences do not apply to statements in or omissions from the Registration
Statement, any Rule 462(b) Registration Statement, or any post-effective
amendment thereto, or the Prospectus, or any amendments or supplements thereto,
made in reliance upon and in conformity with information relating to any
Underwriter furnished to the Company in writing by the Representatives expressly
for use therein. There are no contracts or other documents required to be
described in the Prospectus or to be filed as exhibits to the Registration
Statement which have not been described or filed as required.


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     (b) Offering Materials Furnished to Underwriters. The Company has delivered
to the Representatives one complete manually signed copy of the Registration
Statement and of each consent and certificate of experts filed as a part
thereof, and conformed copies of the Registration Statement (without exhibits)
and preliminary prospectuses and the Prospectus, as amended or supplemented, in
such quantities and at such places as the Representatives have reasonably
requested for each of the Underwriters.

     (c) Distribution of Offering Material By the Company. The Company has not
distributed and will not distribute, prior to the later of the Second Closing
Date (as defined below) and the completion of the Underwriters' distribution of
the Common Shares, any offering material in connection with the offering and
sale of the Common Shares other than a preliminary prospectus, the Prospectus or
the Registration Statement.

     (d) The Underwriting Agreement. This Agreement has been duly authorized,
executed and delivered by, and is a valid and binding agreement of, the Company,
enforceable in accordance with its terms, except as rights to indemnification
hereunder may be limited by applicable law and except as the enforcement hereof
may be limited by bankruptcy, insolvency, reorganization, moratorium or other
similar laws relating to or affecting the rights and remedies of creditors or by
general equitable principles.

     (e) Authorization of the Common Shares. The Common Shares to be purchased
by the Underwriters from the Company have been duly authorized for issuance and
sale pursuant to this Agreement and, when issued and delivered by the Company
pursuant to this Agreement, will be validly issued, fully paid and
nonassessable.

     (f) No Applicable Registration or Other Similar Rights. There are no
persons with registration or other similar rights to have any equity or debt
securities registered for sale under the Registration Statement or included in
the offering contemplated by this Agreement, except for such rights as have been
duly waived.

     (g) No Material Adverse Change. Except as otherwise disclosed in the
Prospectus, subsequent to the respective dates as of which information is given
in the Prospectus: (i) there has been no material adverse change, or any
development that could reasonably be expected to result in a material adverse
change, in the condition, financial or otherwise, or in the earnings, business,
operations or prospects, whether or not arising from transactions in the
ordinary course of business, of the Company and its subsidiaries, considered as
one entity (any such change is called a "Material Adverse Change"); (ii) the
Company and its subsidiaries, considered as one entity, have not incurred any
material liability or obligation, indirect, direct or contingent, not in the
ordinary course of business nor entered into any material transaction or
agreement not in the ordinary course of business; and (iii) there has been no
dividend or distribution of any kind declared, paid or made by the Company or,
except for dividends paid to the Company or other subsidiaries, any of its
subsidiaries on any class of capital stock or repurchase or redemption by the
Company or any of its subsidiaries of any class of capital stock.

     (h) Independent Accountants. PricewaterhouseCoopers LLP, who have expressed
their opinion with respect to the financial statements (which term as used in
this Agreement includes the related notes thereto) filed with the Commission as
a part of the Registration Statement and included in the Prospectus, are
independent public or certified public accountants as required by the Securities
Act.

   
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     (i) Preparation of the Financial Statements. The financial statements filed
with the Commission as a part of the Registration Statement and included in the
Prospectus present fairly the consolidated financial position of the Company and
its subsidiaries as of and at the dates indicated and the results of their
operations and cash flows for the periods specified. Such financial statements
and supporting schedules have been prepared in conformity with generally
accepted accounting principles applied on a consistent basis throughout the
periods involved, except as may be expressly stated in the related notes
thereto. No other financial statements or supporting schedules are required to
be included in the Registration Statement. The financial data set forth in the
Prospectus under the captions "Prospectus Summary--Summary Consolidated
Financial Data", "Selected Consolidated Financial Data" and "Capitalization"
fairly present the information set forth therein on a basis consistent with that
of the audited financial statements contained in the Registration Statement.

   
     (j) Incorporation and Good Standing of the Company and its Subsidiaries.
Each of the Company and its subsidiaries has been duly incorporated and is
validly existing as a corporation in good standing under the laws of the
jurisdiction of its incorporation and has corporate power and authority to own,
lease and operate its properties and to conduct its business as described in the
Prospectus and, in the case of the Company, to enter into and perform its
obligations under this Agreement. Each of the Company and each subsidiary is
duly qualified as a foreign corporation to transact business and is in good
standing in each other jurisdiction in which such qualification is required,
whether by reason of the ownership or leasing of property or the conduct of
business, except for such jurisdictions where the failure to so qualify or to be
in good standing would not, individually or in the aggregate, result in a
Material Adverse Change. All of the issued and outstanding capital stock of each
subsidiary has been duly authorized and validly issued, is fully paid and
nonassessable and immediately prior to the sale of the Common Shares on the
First Closing Date shall be owned by the Company, except to the extent required
to comply with applicable laws, directly or through subsidiaries, free and clear
of any security interest, mortgage, pledge, lien, encumbrance or claim. The
Company does not own or control, directly or indirectly, any corporation,
association or other entity other than the subsidiaries listed in Exhibit 21.1
to the Registration Statement.
    

     (k) Capitalization and Other Capital Stock Matters. After giving effect to
the assumptions set forth in the Prospectus, (i) the authorized, issued and
outstanding capital stock of the Company as of December 31, 1998, was as set
forth in the Prospectus under the caption "Capitalization" (other than for
subsequent issuances, if any, pursuant to employee benefit plans described in
the Prospectus or upon exercise of outstanding options described in the
Prospectus), (ii) the Common Stock (including the Common Shares) conforms in all
material respects to the description thereof contained in the Prospectus. All of
the issued and outstanding shares of Common Stock (including the shares of
Common Stock owned by Selling Stockholders) have been duly authorized and
validly issued, are fully paid and nonassessable and have been issued in
compliance with federal and state securities laws. None of the outstanding
shares of Common Stock were issued in violation of any preemptive rights, rights
of first refusal or other similar rights to subscribe for or purchase securities
of the Company. There are no authorized or outstanding options, warrants,
preemptive rights, rights of first refusal or other rights to purchase, or
equity or debt securities convertible into or exchangeable or exercisable for,
any capital stock of the Company or any of its subsidiaries other than those
accurately described in the Prospectus. The description of the Company's stock
option, stock bonus and other stock plans or arrangements, and the options or
other rights granted thereunder, set forth in the Prospectus accurately and
fairly presents the information required to be shown with respect to such plans,
arrangements, options and rights.

     (l) Stock Exchange Listing. The Common Shares have been approved for
listing on the Nasdaq National Market, subject only to official notice of
issuance.


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     (m) Non-Contravention of Existing Instruments; No Further Authorizations or
Approvals Required. Neither the Company nor any of its subsidiaries is in
violation of its charter or by-laws or is in default (or, with the giving of
notice or lapse of time, would be in default) ("Default") under any indenture,
mortgage, loan or credit agreement, note, contract, franchise, lease or other
instrument to which the Company or any of its subsidiaries is a party or by
which it or any of them may be bound, or to which any of the property or assets
of the Company or any of its subsidiaries is subject (each, an "Existing
Instrument"), except for such Defaults as would not, individually or in the
aggregate, result in a Material Adverse Change. The Company's execution,
delivery and performance of this Agreement and consummation of the transactions
contemplated hereby and by the Prospectus (i) have been duly authorized by all
necessary corporate action and will not result in any violation of the
provisions of the charter or by-laws of the Company or any subsidiary, (ii) will
not conflict with or constitute a breach of, or Default under, or result in the
creation or imposition of any lien, charge or encumbrance upon any property or
assets of the Company or any of its subsidiaries pursuant to, or require the
consent of any other part to, any Existing Instrument, except for such
conflicts, breaches, Defaults, liens, charges or encumbrances as would not,
individually or in the aggregate, result in a Material Adverse Change and (iii)
will not result in any violation of any law, administrative regulation or
administrative or court decree applicable to the Company or any subsidiary. No
consent, approval, authorization or other order of, or registration or filing
with, any court or other governmental or regulatory authority or agency, is
required for the Company's execution, delivery and performance of this Agreement
and consummation of the transactions contemplated hereby and by the Prospectus,
except such as have been obtained or made by the Company and are in full force
and effect under the Securities Act, applicable state securities or blue sky
laws and from the National Association of Securities Dealers, Inc. (the "NASD").

     (n) No Material Actions or Proceedings. There are no legal or governmental
actions, suits or proceedings pending or, to the best of the Company's
knowledge, threatened (i) against or affecting the Company or any of its
subsidiaries, (ii) which has as the subject thereof any officer or director of,
or property owned or leased by, the Company or any of its subsidiaries or (iii)
relating to environmental or discrimination matters, where in any such case (A)
there is a reasonable possibility that such action, suit or proceeding might be
determined adversely to the Company or such subsidiary and (B) any such action,
suit or proceeding, if so determined adversely, would reasonably be expected to
result in a Material Adverse Change or adversely affect the consummation of the
transactions contemplated by this Agreement. No material labor dispute with the
employees of the Company or any of its subsidiaries exists or, to the best of
the Company's knowledge, is threatened or imminent.

     (o) Intellectual Property Rights. The Company and its subsidiaries own or
possess sufficient trademarks, trade names, patent rights, copyrights, licenses,
approvals, trade secrets and other similar rights (collectively, "Intellectual
Property Rights") reasonably necessary to conduct their businesses as now
conducted; and the expected expiration of any of such Intellectual Property
Rights would not result in a Material Adverse Change. Neither the Company nor
any of its subsidiaries has received any notice of infringement or conflict with
asserted Intellectual Property Rights of others, which infringement or conflict,
if the subject of an unfavorable decision, would result in a Material Adverse
Change.

     (p) All Necessary Permits, etc. The Company and each subsidiary possess
such valid and current certificates, authorizations or permits issued by the
appropriate state, federal or foreign regulatory agencies or bodies necessary to
conduct their respective businesses (other than any the absence of which would
not, singly or in the aggregate, result in a Material Adverse Change, and
neither the Company nor any subsidiary has received any notice of proceedings
relating to the revocation or modification of, or non-compliance with, any such
certificate, authorization or permit which, singly or in


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the aggregate, if the subject of an unfavorable decision, ruling or finding,
could result in a Material Adverse Change.

     (q) Title to Properties. The Company and each of its subsidiaries has good
and marketable title to all the properties and assets reflected as owned in the
financial statements referred to in Section (i) above (or elsewhere in the
Prospectus), in each case, except as disclosed in the Prospectus, free and clear
of any security interests, mortgages, liens, encumbrances, equities, claims and
other defects, except such as do not materially and adversely affect the value
of such property and do not materially interfere with the use made or proposed
to be made of such property by the Company or such subsidiary. The real
property, improvements, equipment and personal property held under lease by the
Company or any subsidiary are held under valid and enforceable leases, with such
exceptions as are not material and do not materially interfere with the use made
or proposed to be made of such real property, improvements, equipment or
personal property by the Company or such subsidiary.

     (r) Tax Law Compliance. The Company and its subsidiaries have filed all
necessary federal, state and foreign income and franchise tax returns or have
properly requested extensions thereof and have paid all taxes required to be
paid by any of them and, if due and payable, any related or similar assessment,
fine or penalty levied against any of them except as may be being contested in
good faith and by appropriate proceedings. The Company has made adequate
charges, accruals and reserves in the applicable financial statements referred
to in Section 1(i) above in respect of all federal, state and foreign income and
franchise taxes for all periods as to which the tax liability of the Company or
any of its subsidiaries has not been finally determined.

     (s) Company Not an "Investment Company". The Company has been advised of
the rules and requirements under the Investment Company Act of 1940, as amended
(the "Investment Company Act"). The Company is not, and after receipt of payment
for the Common Shares will not be, an "investment company" within the meaning of
Investment Company Act and will conduct its business in a manner so that it will
not become subject to the Investment Company Act.

     (t) Insurance. Each of the Company and its subsidiaries is insured by
recognized and reputable institutions with policies in such amounts and with
such deductibles and covering such risks as the Company has reasonably deemed
adequate and customary for their businesses including, but not limited to,
policies covering real and personal property owned or leased by the Company and
its subsidiaries against theft, damage, destruction, and acts of vandalism. The
Company has no reason to believe that it or any subsidiary will not be able (i)
to renew its existing insurance coverage as and when such policies expire or
(ii) to obtain comparable coverage from similar institutions as may be necessary
or appropriate to conduct its business as now conducted and at a cost that would
not result in a Material Adverse Change. Neither of the Company nor any
subsidiary has been denied any insurance coverage which it has sought or for
which it has applied.

     (u) No Price Stabilization or Manipulation. The Company has not taken and
will not take, directly or indirectly, any action designed to or that might be
reasonably expected to cause or result in stabilization or manipulation of the
price of the Common Stock to facilitate the sale or resale of the Common Shares.

     (v) Related Party Transactions. There are no business relationships or
related-party transactions involving the Company or any subsidiary or any other
person required to be described in the Prospectus which have not been described
as required.


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     (w) No Unlawful Contributions or Other Payments. Neither the Company nor
any of its subsidiaries nor, to the best of the Company's knowledge, any
employee or agent of the Company or any subsidiary, has made any contribution or
other payment to any official of, or candidate for, any federal, state or
foreign office in violation of any law or of the character required to be
disclosed in the Prospectus.

          Any certificate signed by an officer of the Company and delivered to 
the Representatives or to counsel for the Underwriters shall be deemed to be a
representation and warranty by the Company to each Underwriter as to the matters
set forth therein.

Section 1B. Representations and Warranties of the Selling Stockholders. Each
Selling Stockholder represents, warrants and covenants to each Underwriter as
follows:

     (a) The Underwriting Agreement. This Agreement has been duly executed and 
delivered by or on behalf of such Selling Stockholder and is a valid and binding
agreement of such Selling Stockholder, enforceable in accordance with its terms,
except as rights to indemnification hereunder may be limited by applicable law
and except as the enforcement hereof may be limited by bankruptcy, insolvency,
reorganization, moratorium or other similar laws relating to or affecting the
rights and remedies of creditors or by general equitable principles.

   
     (b) The Custody Agreement and Power of Attorney. Each of the (i) Custody
Agreement signed by such Selling Stockholder and BankBoston, N.A., as custodian
(the "Custodian"), relating to the deposit of the Common Shares to be sold by
such Selling Stockholder (the "Custody Agreement") and (ii) Power of Attorney
appointing certain individuals named therein as such Selling Stockholder's
attorneys-in-fact (each, an "Attorney-in-Fact") to the extent set forth therein
relating to the transactions contemplated hereby and by the Prospectus (the
"Power of Attorney"), of such Selling Stockholder has been duly executed and
delivered by such Selling Stockholder and is a valid and binding agreement of
such Selling Stockholder, enforceable in accordance with its terms, except as
rights to indemnification thereunder may be limited by applicable law and except
as the enforcement thereof may be limited by bankruptcy, insolvency,
reorganization, moratorium or other similar laws relating to or affecting the
rights and remedies of creditors or by general equitable principles.
    

     (c) Title to Common Shares to be Sold; All Authorizations Obtained. Such
Selling Stockholder has, and on the First Closing Date and the Second Closing
Date (as defined below) will have, good and valid title to all of the Common
Shares which may be sold by such Selling Stockholder pursuant to this Agreement
on such date and the legal right and power, and all authorizations and approvals
required by law and under its trust agreement or other organizational documents
to enter into this Agreement and its Custody Agreement and Power of Attorney, to
sell, transfer and deliver all of the Common Shares which may be sold by such
Selling Stockholder pursuant to this Agreement and to comply with its other
obligations hereunder and thereunder.

     (d) Delivery of the Common Shares to be Sold. Delivery of the Common Shares
which are sold by such Selling Stockholder pursuant to this Agreement will pass
good and valid title to such Common Shares, free and clear of any security
interest, mortgage, pledge, lien, encumbrance or other claim.


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     (e) Non-Contravention; No Further Authorizations or Approvals Required. The
execution and delivery by such Selling Stockholder of, and the performance by
such Selling Stockholder of its obligations under, this Agreement, the Custody
Agreement and the Power of Attorney will not contravene or conflict with, result
in a breach of, or constitute a Default under, or require the consent of any
other party to, the trust agreement of such Selling Stockholder or any other
agreement or instrument to which such Selling Stockholder is a party or by which
it is bound or under which it is entitled to any right or benefit, any provision
of applicable law or any judgment, order, decree or regulation applicable to
such Selling Stockholder of any court, regulatory body, administrative agency,
governmental body or arbitrator having jurisdiction over such Selling
Stockholder. No consent, approval, authorization or other order of, or
registration or filing with, any court or other governmental authority or
agency, is required for the consummation by such Selling Stockholder of the
transactions contemplated in this Agreement, except such as have been obtained
or made and are in full force and effect under the Securities Act, applicable
state securities or blue sky laws and from the NASD.

     (f) No Registration or Other Similar Rights. Such Selling Stockholder does
not have any registration or other similar rights to have any equity or debt
securities registered for sale by the Company under the Registration Statement
or included in the offering contemplated by this Agreement, except for such
rights as are described in the Prospectus under "Shares Eligible for Future
Sale."

     (g) No Further Consents, etc. No consent, approval or waiver is required
under any instrument or agreement to which such Selling Stockholder is a party
or by which it is bound or under which it is entitled to any right or benefit,
in connection with the offering, sale or purchase by the Underwriters of any of
the Common Shares which may be sold by such Selling Stockholder under this
Agreement or the consummation by such Selling Stockholder of any of the other
transactions contemplated hereby.

   
     (h) Disclosure Made by Such Selling Stockholder in the Prospectus. All
information furnished by or on behalf of such Selling Stockholder in writing
expressly for use in the Registration Statement and Prospectus is, and on the
First Closing Date and the Second Closing Date will be, true, correct, and
complete in all material respects, and does not, and on the First Closing Date
and the Second Closing Date will not, contain any untrue statement of a material
fact or omit to state any material fact necessary to make such information not
misleading. Such Selling Stockholder confirms as accurate the number of shares
of Common Stock set forth opposite such Selling Stockholder's name in Schedule B
to this Agreement (prior to giving effect to the sale of the Common Shares).
    

     (i) No Price Stabilization or Manipulation. Such Selling Stockholder has
not taken and will not take, directly or indirectly, any action designed to or
that might be reasonably expected to cause or result in stabilization or
manipulation of the price of the Common Stock to facilitate the sale or resale
of the Common Shares.

     (j) Confirmation of Company Representations and Warranties. Such Selling
Stockholder has no reason to believe that the representations and warranties of
the Company contained in Section 1A hereof are not true and correct, is familiar
with the Registration Statement and the Prospectus and has no knowledge of any
material fact, condition or information not disclosed in the Registration
Statement or the Prospectus which has had or may have a Material Adverse Effect
and is not prompted to sell shares of Common Stock by any information concerning
the Company which is not set forth in the Registration Statement and the
Prospectus.


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     Any certificate signed by or on behalf of any Selling Stockholder and
delivered to the Representatives or to counsel for the Underwriters shall be
deemed to be a representation and warranty by such Selling Stockholder to each
Underwriter as to the matters covered thereby.

Section 2. Purchase, Sale and Delivery of the Common Shares.

          The Firm Common Shares. Upon the terms set forth herein, (i) the
Company agrees to issue and sell to the several Underwriters an aggregate of
6,000,000 Firm Common Shares and (ii) the Selling Stockholders agree to sell to
the several Underwriters an aggregate of 500,000 Firm Common Shares, each
Selling Stockholder selling the number of Firm Common Shares set forth opposite
such Selling Stockholder's name on Schedule B. On the basis of the
representations, warranties and agreements herein contained, and upon the terms
but subject to the conditions herein set forth, the Underwriters agree,
severally and not jointly, to purchase from the Company and the Selling
Stockholders the respective number of Firm Common Shares set forth opposite
their names on Schedule A. The purchase price per Firm Common Share to be paid
by the several Underwriters to the Company and the Selling Stockholders shall be
$[___] per share.

          The First Closing Date. Delivery of certificates for the Firm Common
Shares to be purchased by the Underwriters and payment therefor shall be made at
the offices of NMS, 600 Montgomery Street, San Francisco, California (or such
other place as may be agreed to by the Company and the Representatives) at 6:00
a.m. San Francisco time, on [___], or such other time and date not later than
10:30 a.m. San Francisco time, on [___] as the Representatives shall designate
by notice to the Company (the time and date of such closing are called the
"First Closing Date"). The Company and the Selling Stockholders hereby
acknowledge that circumstances under which the Representatives may provide
notice to postpone the First Closing Date as originally scheduled include, but
are in no way limited to, any determination by the Company, the Selling
Stockholders or the Representatives to recirculate to the public copies of an
amended or supplemented Prospectus or a delay as contemplated by the provisions
of Section 10.

          The Optional Common Shares; the Second Closing Date. In addition, on
the basis of the representations, warranties and agreements herein contained,
and upon the terms but subject to the conditions herein set forth, the Company
hereby grants an option to the several Underwriters to purchase, severally and
not jointly, up to an aggregate of 975,000 Optional Common Shares from the
Company at the purchase price per share to be paid by the Underwriters for the
Firm Common Shares. The option granted hereunder is for use by the Underwriters
solely in covering any over-allotments in connection with the sale and
distribution of the Firm Common Shares. The option granted hereunder may be
exercised at any time (but not more than once) upon notice by the
Representatives to the Company, which notice may be given at any time within 30
days from the date of this Agreement. Such notice shall set forth (i) the
aggregate number of Optional Common Shares as to which the Underwriters are
exercising the option, (ii) the names and denominations in which the
certificates for the Optional Common Shares are to be registered and (iii) the
time, date and place at which such certificates will be delivered (which time
and date may be simultaneous with, but not earlier than, the First Closing Date;
and in such case the term "First Closing Date" shall refer to the time and date
of delivery of certificates for the Firm Common Shares and the Optional Common
Shares). Such time and date of delivery, if subsequent to the First Closing
Date, is called the "Second Closing Date" and shall be determined by the
Representatives and shall not be earlier than three nor later than five full
business days after delivery of such notice of exercise. If any Optional Common
Shares are to be purchased, (a) each Underwriter


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<PAGE>   11

agrees, severally and not jointly, to purchase the number of Optional Common
Shares (subject to such adjustments to eliminate fractional shares as the
Representatives may determine) that bears the same proportion to the total
number of Optional Common Shares to be purchased as the number of Firm Common
Shares set forth on Schedule A opposite the name of such Underwriter bears to
the total number of Firm Common Shares and (b) the Company agrees to sell the
number of Optional Common Shares (subject to such adjustments to eliminate
fractional shares as the Representatives may determine) that bears the same
proportion to the total number of Optional Common Shares to be sold as the
number of Optional Common Shares to be sold by the Company as set forth in the
paragraph "Introductory" of this Agreement bears to the total number of Optional
Common Shares. The Representatives may cancel the option at any time prior to
its expiration by giving written notice of such cancellation to the Company.

          Public Offering of the Common Shares. The Representatives hereby
advise the Company and the Selling Stockholders that the Underwriters intend to
offer for sale to the public, as described in the Prospectus, their respective
portions of the Common Shares as soon after this Agreement has been executed and
the Registration Statement has been declared effective as the Representatives,
in their sole judgment, have determined is advisable and practicable.

          Payment for the Common Shares. Payment for the Common Shares. Payment
for the Common Shares to be sold by the Company shall be made at the First
Closing Date (and, if applicable, at the Second Closing Date) by wire transfer
of immediately available funds to the order of the Company. Payment for the
Common Shares to be sold by the Selling Stockholders shall be made at the First
Closing Date by wire transfer of immediately available funds to the order of the
Custodian.

          It is understood that the Representatives have been authorized, for
their own account and the accounts of the several Underwriters, to accept
delivery of and receipt for, and make payment of the purchase price for, the
Firm Common Shares and any Optional Common Shares the Underwriters have agreed
to purchase. NMS, individually and not as the Representatives of the
Underwriters, may (but shall not be obligated to) make payment for any Common
Shares to be purchased by any Underwriter whose funds shall not have been
received by the Representatives by the First Closing Date or the Second Closing
Date, as the case may be, for the account of such Underwriter, but any such
payment shall not relieve such Underwriter from any of its obligations under
this Agreement.

          Each Selling Stockholder hereby agrees that (i) it will pay all stock
transfer taxes, stamp duties and other similar taxes, if any, payable upon the
sale or delivery of the Common Shares to be sold by such Selling Stockholder to
the several Underwriters, or otherwise in connection with the performance of
such Selling Stockholder's obligations hereunder and (ii) the Custodian is
authorized to deduct for such payment any such amounts from the proceeds to such
Selling Stockholder hereunder and to hold such amounts for the account of such
Selling Stockholder with the Custodian under the Custody Agreement.

          Delivery of the Common Shares. The Company and the Selling
Stockholders shall deliver, or cause to be delivered, to the Representatives for
the accounts of the several Underwriters certificates for the Firm Common Shares
to be sold by them at the First Closing Date, against the irrevocable release of
a wire transfer of immediately available funds for the amount of the purchase
price therefor. The Company shall also deliver, or cause to be delivered, to the
Representatives for the accounts of the several Underwriters, certificates for
the Optional Common Shares the Underwriters have agreed to purchase from it on
the First Closing Date or the Second Closing Date, as the case may be,


                                       10


<PAGE>   12


against the irrevocable release of a wire transfer of immediately available
funds for the amount of the purchase price therefor. The certificates for the
Common Shares shall be in definitive form and registered in such names and
denominations as the Representatives shall have requested at least two full
business days prior to the First Closing Date (or the Second Closing Date, as
the case may be) and shall be made available for inspection on the business day
preceding the First Closing Date (or the Second Closing Date, as the case may
be) at a location in New York City as the Representatives may designate. Time
shall be of the essence, and delivery at the time and place specified in this
Agreement is a further condition to the obligations of the Underwriters.

          Delivery of Prospectus to the Underwriters. Not later than 12:00 p.m.
on the second business day following the date the Common Shares are released by
the Underwriters for sale to the public, the Company shall deliver or cause to
be delivered copies of the Prospectus in such quantities and at such places as
the Representatives shall request.

Section 3A. Additional Covenants of the Company. The Company further covenants
and agrees with each Underwriter as follows:

     (a) Representatives' Review of Proposed Amendments and Supplements. During
such period beginning on the date hereof and ending on the later of the First
Closing Date or such date, as in the opinion of counsel for the Underwriters,
the Prospectus is no longer required by law to be delivered in connection with
sales by an Underwriter or dealer (the "Prospectus Delivery Period"), prior to
amending or supplementing the Registration Statement (including any registration
statement filed under Rule 462(b) under the Securities Act) or the Prospectus,
the Company shall furnish to the Representatives for review a copy of each such
proposed amendment or supplement, and the Company shall not file any such
proposed amendment or supplement to which the Representatives reasonably object.

     (b) Securities Act Compliance. After the date of this Agreement, the
Company shall promptly advise the Representatives in writing (i) of the receipt
of any comments of, or requests for additional or supplemental information from,
the Commission, (ii) of the time and date of any filing of any post-effective
amendment to the Registration Statement or any amendment or supplement to any
preliminary prospectus or the Prospectus, (iii) of the time and date that any
post-effective amendment to the Registration Statement becomes effective and
(iv) of the issuance by the Commission of any stop order suspending the
effectiveness of the Registration Statement or any post-effective amendment
thereto or of any order preventing or suspending the use of any preliminary
prospectus or the Prospectus, or of any proceedings to remove, suspend or
terminate from listing or quotation the Common Stock from any securities
exchange upon which it is listed for trading or included or designated for
quotation, or of the threatening or initiation of any proceedings for any of
such purposes. If the Commission shall enter any such stop order at any time,
the Company will use its best efforts to obtain the lifting of such order at the
earliest possible moment. Additionally, the Company agrees that it shall comply
with the provisions of Rules 424(b), 430A and 434, as applicable, under the
Securities Act and will use its reasonable efforts to confirm that any filings
made by the Company under such Rule 424(b) were received in a timely manner by
the Commission.

     (c) Amendments and Supplements to the Prospectus and Other Securities Act
Matters. If, during the Prospectus Delivery Period, any event shall occur or
condition exist as a result of which it is necessary to amend or supplement the
Prospectus in order to make the statements therein, in the light of the
circumstances when the Prospectus is delivered to a purchaser, not misleading,
or if in the opinion of


                                       11


<PAGE>   13


the Representatives or counsel for the Underwriters it is otherwise necessary to
amend or supplement the Prospectus to comply with law, the Company agrees to
promptly prepare (subject to Section 3A(a) hereof), file with the Commission and
furnish at its own expense to the Underwriters and to dealers, amendments or
supplements to the Prospectus so that the statements in the Prospectus as so
amended or supplemented will not, in the light of the circumstances when the
Prospectus is delivered to a purchaser, be misleading or so that the Prospectus,
as amended or supplemented, will comply with law.

     (d) Copies of any Amendments and Supplements to the Prospectus. The Company
agrees to furnish the Representatives, without charge, during the Prospectus
Delivery Period, as many copies of the Prospectus and any amendments and
supplements thereto as the Representatives may request.

     (e) Blue Sky Compliance. The Company shall cooperate with the
Representatives and counsel for the Underwriters to qualify or register the
Common Shares for sale under (or obtain exemptions from the application of) the
state securities or blue sky laws or Canadian provincial Securities laws of
those jurisdictions designated by the Representatives, shall comply with such
laws and shall continue such qualifications, registrations and exemptions in
effect so long as required for the distribution of the Common Shares. The
Company shall not be required to qualify as a foreign corporation or to take any
action that would subject it to general service of process in any such
jurisdiction where it is not presently qualified or where it would be subject to
taxation as a foreign corporation. The Company will advise the Representatives
promptly of the suspension of the qualification or registration of (or any such
exemption relating to) the Common Shares for offering, sale or trading in any
jurisdiction or any initiation or threat of any proceeding for any such purpose,
and in the event of the issuance of any order suspending such qualification,
registration or exemption, the Company shall use its best efforts to obtain the
withdrawal thereof at the earliest possible moment.

     (f) Use of Proceeds. The Company shall apply the net proceeds from the sale
of the Common Shares sold by it in the manner described under the caption "Use
of Proceeds" in the Prospectus.

     (g) Transfer Agent. The Company shall engage and maintain, at its expense,
a registrar and transfer agent for the Common Stock.

   
     (h) Earnings Statement. As soon as practicable, the Company will make
generally available to its security holders and to the Representatives an
earnings statement (which need not be audited) covering the twelve-month period
ending March 31, 2000 that satisfies the provisions of Section 11(a) of the
Securities Act.
    

     (i) Periodic Reporting Obligations. During the Prospectus Delivery Period
the Company shall file, on a timely basis, with the Commission and the Nasdaq
National Market all reports and documents required to be filed under the
Exchange Act.

     (j) Agreement Not To Offer or Sell Additional Securities. During the period
of 180 days following the date of the Prospectus, the Company will not, without
the prior written consent of NMS (which consent may be withheld at the sole
discretion of NMS), directly or indirectly, sell, offer, contract or grant any
option to sell, pledge, transfer or establish an open "put equivalent position"
within the meaning of Rule 16a-1(h) under the Exchange Act, or otherwise dispose
of or transfer, or announce the offering of, or file any registration statement
under the Securities Act in respect of, any shares of Common Stock, options or
warrants to acquire shares of the Common Stock or securities exchangeable


                                       12


<PAGE>   14


or exercisable for or convertible into shares of Common Stock (other than as
contemplated by this Agreement with respect to the Common Shares); provided,
however, that the Company may issue shares of its Common Stock or options to
purchase its Common Stock, or Common Stock upon exercise of options, pursuant to
any stock option, stock bonus or other stock plan or arrangement described in
the Prospectus, (ii) file one or more Registration Statements on Form S-8, and
(iii) issue shares in connection with any acquisition if recipients agree in
writing not to sell, offer, dispose of or otherwise transfer any such shares
during such 180 day period without the prior written consent of NMS (which
consent may be withheld at the sole discretion of the NMS).

   
     (k) Future Reports to the Representatives. During the period of five years
hereafter the Company will furnish to the Representatives at Two International
Place, Boston, MA 02110  Attention: Timothy H. Harned, Managing Director: (i) as
soon as practicable after the end of each fiscal year, copies of the Annual
Report of the Company containing the balance sheet of the Company as of the
close of such fiscal year and statements of income, stockholders' equity and
cash flows for the year then ended and the opinion thereon of the Company's
independent public or certified public accountants; (ii) as soon as practicable
after the filing thereof, copies of each proxy statement, Annual Report on Form
10-K, Quarterly Report on Form 10-Q, Current Report on Form 8-K or other report
filed by the Company with the Commission, the NASD or any securities exchange;
and (iii) as soon as available, copies of any report or communication of the
Company mailed generally to holders of its capital stock.
    

Section 3B. Covenants of the Selling Stockholders. Each Selling Stockholder
further covenants and agrees with each Underwriter:

     (a) Agreement Not to Offer or Sell Additional Securities. Such Selling
Stockholder will not, without the prior written consent of NMS (which consent
may be withheld in its sole discretion), directly or indirectly, sell, offer,
contract or grant any option to sell (including without limitation any short
sale), pledge, transfer, establish an open "put equivalent position" within the
meaning of Rule 16a-1(h) under the Exchange Act, or otherwise dispose of any
shares of Common Stock, options or warrants to acquire shares of Common Stock,
or securities exchangeable or exercisable for or convertible into shares of
Common Stock currently or hereafter owned either of record or beneficially (as
defined in Rule 13d-3 under Securities Exchange Act of 1934, as amended) by the
undersigned, or publicly announce the undersigned's intention to do any of the
foregoing, for a period commencing on the date hereof and continuing through the
close of trading on the date 180 days after the date of the Prospectus.

     (b) Delivery of Forms W-8 and W-9 . To deliver to the Representative prior
to the First Closing Date a properly completed and executed United States
Treasury Department Form W-8 (if the Selling Stockholder is a non-United States
person) or Form W-9 (if the Selling Stockholder is a United States Person).

Section 4. Payment of Expenses. The Company agrees to pay all costs, fees and
expenses incurred in connection with the performance of its obligations
hereunder and in connection with the transactions contemplated hereby, including
without limitation (i) all expenses incident to the issuance and delivery of the
Common Shares (including all printing and engraving costs), (ii) all fees and
expenses of the registrar and transfer agent of the Common Stock, (iii) all
necessary issue, transfer and other stamp taxes in connection with the issuance
and sale of the Common Shares to the Underwriters, (iv) all fees and


                                       13


<PAGE>   15


   
expenses of the Company's counsel, independent public or certified public
accountants and other advisors, (v) all costs and expenses incurred in
connection with the preparation, printing, filing, shipping and distribution of
the Registration Statement (including financial statements, exhibits, schedules,
consents and certificates of experts), each preliminary prospectus and the
Prospectus, and all amendments and supplements thereto, and this Agreement, (vi)
all filing fees, attorneys' fees and expenses incurred by the Company or the
Underwriters in connection with qualifying or registering (or obtaining
exemptions from the qualification or registration of) all or any part of the
Common Shares for offer and sale under the state securities or blue sky laws or
the provincial securities laws of Canada, and, if requested by the
Representatives, preparing and printing a "Blue Sky Survey" or memorandum, and
any supplements thereto, advising the Underwriters of such qualifications,
registrations and exemptions, subject to a maximum fee of $7,500 (vii) the
filing fees incident to, and the reasonable fees and expenses of counsel for the
Underwriters in connection with, the NASD's review and approval of the
Underwriters' participation in the offering and distribution of the Common
Shares, (viii) the fees and expenses associated with listing the Common Shares
on the Nasdaq National Market, and (ix) all other fees, costs and expenses
referred to in Item 13 of Part II of the Registration Statement. Except as
provided in this Section 4, Section 6, Section 8 and Section 9 hereof, the
Underwriters shall pay their own expenses, including the fees and disbursements
of their counsel.
    

          The Selling Stockholders further agree with each Underwriter to pay
(directly or by reimbursement) all fees and expenses incident to the performance
of their obligations under this Agreement which are not otherwise specifically
provided for herein, including but not limited to (i) fees and expenses of
counsel and other advisors for such Selling Stockholders, (ii) fees and expenses
of the Custodian and (iii) expenses and taxes incident to the sale and delivery
of the Common Shares to be sold by such Selling Stockholders to the Underwriters
hereunder (which taxes, if any, may be deducted by the Custodian under the
provisions of Section 2 of this Agreement).

          This Section 4 shall not affect or modify any separate, valid
agreement relating to the allocation of payment of expenses between the Company,
on the one hand, and the Selling Stockholders, on the other hand.

Section 5. Conditions of the Obligations of the Underwriters. The obligations of
the several Underwriters to purchase and pay for the Common Shares as provided
herein on the First Closing Date and, with respect to the Optional Common
Shares, the Second Closing Date, shall be subject to the accuracy of the
representations and warranties on the part of the Company and the Selling
Stockholders set forth in Sections 1A and 1B hereof as of the date hereof and as
of the First Closing Date as though then made and, with respect to the Optional
Common Shares, as of the Second Closing Date as though then made, to the timely
performance by the Company and the Selling Stockholders of their covenants and
other obligations hereunder, and to each of the following additional conditions:

   
     (a) Accountants' Comfort Letter. On the date hereof, the Representatives
shall have received from PricewaterhouseCoopers LLP, independent public or
certified public accountants for the Company, a letter dated the date hereof
addressed to the Underwriters, in form and substance satisfactory to the
Representatives, containing statements and information of the type ordinarily
included in accountant's "comfort letters" to underwriters, delivered according
to Statement of Auditing Standards No. 72 (or any successor bulletin), with
respect to the audited and unaudited financial statements and certain financial
information contained in the Registration Statement and the Prospectus (and the
Representatives shall have received an additional five conformed copies of such
accountants' letter for each of the several Underwriters).
    


                                       14


<PAGE>   16


     (b) Compliance with Registration Requirements; No Stop Order; No Objection
from NASD. For the period from and after effectiveness of this Agreement and
prior to the First Closing Date and, with respect to the Optional Common Shares,
the Second Closing Date:

          (i) the Company shall have filed the Prospectus with the Commission
(including the information required by Rule 430A under the Securities Act) in
the manner and within the time period required by Rule 424(b) under the
Securities Act; or the Company shall have filed a post-effective amendment to
the Registration Statement containing the information required by such Rule
430A, and such post-effective amendment shall have become effective; or, if the
Company elected to rely upon Rule 434 under the Securities Act and obtained the
Representatives' consent thereto, the Company shall have filed a Term Sheet with
the Commission in the manner and within the time period required by such Rule
424(b);

          (ii) no stop order suspending the effectiveness of the Registration
Statement, any Rule 462(b) Registration Statement, or any post-effective
amendment to the Registration Statement, shall be in effect and no proceedings
for such purpose shall have been instituted or threatened by the Commission; and

          (iii) the NASD shall have raised no objection to the fairness and
reasonableness of the underwriting terms and arrangements.

     (c) No Material Adverse Change or Ratings Agency Change. For the period
from and after the date of this Agreement and prior to the First Closing Date
and, with respect to the Optional Common Shares, the Second Closing Date:

          (i) in the judgment of the Representatives there shall not have
occurred any Material Adverse Change; and

          (ii) there shall not have occurred any downgrading, nor shall any
notice have been given of any intended or potential downgrading or of any review
for a possible change that does not indicate the direction of the possible
change, in the rating accorded any securities of the Company or any of its
subsidiaries by any "nationally recognized statistical rating organization" as
such term is defined for purposes of Rule 436(g)(2) under the Securities Act.

   
     (d) Opinion of Counsel for the Company. On each of the First Closing Date
and the Second Closing Date the Representatives shall have received the
favorable opinion of Hale and Dorr, counsel for the Company, dated as of such
Closing Date, reasonably satisfactory to the Representatives with respect to the
matters set forth in Exhibit A-1 attached hereto. On each of the First Closing
Date and the Second Closing Date the Representatives shall have received the
favorable opinion of Hill & Barlow, counsel for the Company, dated as of such
Closing Date, reasonably satisfactory to the Representatives with respect to the
matters set forth in Exhibit A-2 attached hereto. In each case, the
Representatives shall have received an additional five conformed copies of such
counsel's legal opinions for each of the several Underwriters.
    

     (e) Opinion of Counsel for the Underwriters. On each of the First Closing
Date and the Second Closing Date the Representatives shall have received the
favorable opinion of Ropes & Gray, counsel for the Underwriters, dated as of
such Closing Date, with respect to the matters set forth in


                                       15


<PAGE>   17


   
paragraphs (i), (viii), (ix) and the next-to-last paragraph of Exhibit A-1 (and
the Representatives shall have received an additional five conformed copies of
such counsel's legal opinion for each of the several Underwriters).
    

     (f) Officers' Certificate. On each of the First Closing Date and the Second
Closing Date the Representatives shall have received a written certificate
executed by the Chairman of the Board, Chief Executive Officer or President of
the Company and the Chief Financial Officer or Chief Accounting Officer of the
Company, dated as of such Closing Date, to the effect set forth in subsections
(b)(ii) and (c)(ii) of this Section 5, and further to the effect that:

          (i) for the period from and after the date of this Agreement and prior
to such Closing Date, there has not occurred any Material Adverse Change;

          (ii) the representations, warranties and covenants of the Company set
forth in Section 1A of this Agreement are true and correct with the same force
and effect as though expressly made on and as of such Closing Date; and

          (iii) the Company has complied with all the agreements and satisfied
all the conditions on its part to be performed or satisfied at or prior to such
Closing Date.

   
     (g) Bring-down Comfort Letter. On each of the First Closing Date and the
Second Closing Date the Representatives shall have received from
PricewaterhouseCoopers LLP, independent public or certified public accountants
for the Company, a letter dated such date, in form and substance satisfactory to
the Representatives, to the effect that they reaffirm the statements made in the
letter furnished by them pursuant to subsection (a) of this Section 5, except
that the specified date referred to therein for the carrying out of procedures
shall be no more than three business days prior to the First Closing Date or
Second Closing Date, as the case may be (and the Representatives shall have
received an additional five conformed copies of such accountants' letter for
each of the several Underwriters).
    

     (h) Lock-Up Agreement from Stockholders and Optionholders of the Company.
On the date hereof, the Company shall have furnished to the Representatives an
agreement in the form of Exhibit B hereto from those stockholders and
optionholders of the Company as agreed among the Company and the
Representatives, and such agreement shall be in full force and effect on each of
the First Closing Date and the Second Closing Date.

   
     (i) Opinion of Counsel for the Selling Stockholders. On the First Closing
Date the Representatives shall have received the favorable opinion of Hill &
Barlow, counsel for the Selling Stockholders, dated as of such Closing Date, the
form of which is attached as Exhibit A-3 (and the Representatives shall have
received an additional five conformed copies of such counsel's legal opinion for
each of the several Underwriters).
    

     (j) Selling Stockholders' Certificate. On the First Closing Date the
Representatives shall received a written certificate executed by each Selling
Stockholder, dated as of such Closing Date, to the effect that:

          (i) the representations, warranties and covenants of such Selling
Stockholder set forth in Section 1B of this Agreement are true and correct with
the same force and effect as though expressly made by such Selling Stockholder
on and as of such Closing Date; and


                                       16


<PAGE>   18


          (ii) such Selling Stockholder has complied with all the agreements and
satisfied all the conditions on its part to be performed or satisfied at or
prior to such Closing Date.

     (k) Selling Stockholders' Documents. On the date hereof, the Company and
the Selling Stockholders shall have furnished for review by the Representatives
copies of the Powers of Attorney and Custody Agreements executed by each of the
Selling Stockholders and such further information, certificates and documents as
the Representatives may reasonably request.

     (l) Additional Documents. On or before each of the First Closing Date and
the Second Closing Date, the Representatives and counsel for the Underwriters
shall have received such information, documents and opinions as they may
reasonably require for the purposes of enabling them to pass upon the issuance
and sale of the Common Shares as contemplated herein, or in order to evidence
the accuracy of any of the representations and warranties, or the satisfaction
of any of the conditions or agreements, herein contained.

     If any condition specified in this Section 5 is not satisfied when and as
required to be satisfied, this Agreement may be terminated by the
Representatives by notice to the Company and the Selling Stockholders at any
time on or prior to the First Closing Date and, with respect to the Optional
Common Shares, at any time prior to the Second Closing Date, which termination
shall be without liability on the part of any party to any other party, except
that Section 4, Section 6, Section 8 and Section 9 shall at all times be
effective and shall survive such termination.

Section 6. Reimbursement of Underwriters' Expenses. If this Agreement is
terminated by the Representatives pursuant to Section 5, Section 7, or Section
11 (iv) or 11(v), or if the sale to the Underwriters of the Common Shares on the
First Closing Date is not consummated because of any refusal, inability or
failure on the part of the Company or the Selling Stockholders to perform any
agreement herein or to comply with any provision hereof, the Company agrees to
reimburse the Representatives and the other Underwriters (or such Underwriters
as have terminated this Agreement with respect to themselves), severally, upon
demand for all out-of-pocket expenses that shall have been reasonably incurred
by the Representatives and the Underwriters in connection with the proposed
purchase and the offering and sale of the Common Shares, including but not
limited to fees and disbursements of counsel, printing expenses, travel
expenses, postage, facsimile and telephone charges.

Section 7.  Effectiveness of this Agreement.

          This Agreement shall not become effective until the later of (i) the
execution of this Agreement by the parties hereto and (ii) notification by the
Commission to the Company or the Representatives of the effectiveness of the
Registration Statement under the Securities Act.

          Prior to such effectiveness, this Agreement may be terminated by any
party by notice to each of the other parties hereto, and any such termination
shall be without liability on the part of (a) the Company or the Selling
Stockholders to any Underwriter, except that the Company and the Selling
Stockholders shall be obligated to reimburse the expenses of the Representatives
and the Underwriters pursuant to Sections 4 and 6 hereof, (b) of any Underwriter
to the Company or the Selling Stockholders, or (c) of any party hereto to any
other party except that the provisions of Section 8 and Section 9 shall at all
times be effective and shall survive such termination.


                                       17


<PAGE>   19


Section 8. Indemnification.

   
        (a) Indemnification of the Underwriters. (1) The Company agrees to
indemnify and hold harmless each Underwriter, its officers and employees, and
each person, if any, who controls any Underwriter within the meaning of the
Securities Act and the Exchange Act against any loss, claim, damage, liability
or expense, as incurred, to which such Underwriter or such controlling person
may become subject, under the Securities Act, the Exchange Act or other federal
or state statutory law or regulation, or at common law or otherwise (including
in settlement of any litigation, if such settlement is effected with the written
consent of the Company), insofar as such loss, claim, damage, liability or
expense (or actions in respect thereof as contemplated below) arises out of or
is based (i) upon any untrue statement or alleged untrue statement of a material
fact contained in the Registration Statement, or any amendment thereto,
including any information deemed to be a part thereof pursuant to Rule 430A or
Rule 434 under the Securities Act, or the omission or alleged omission therefrom
of a material fact required to be stated therein or necessary to make the
statements therein not misleading; or (ii) upon any untrue statement or alleged
untrue statement of a material fact contained in any preliminary prospectus or
the Prospectus (or any amendment or supplement thereto), or the omission or
alleged omission therefrom of a material fact necessary in order to make the
statements therein, in the light of the circumstances under which they were
made, not misleading; or (iii) in whole or in part upon any inaccuracy in the
representations and warranties of the Company contained herein; or (iv) in whole
or in part upon any failure of the Company to perform its obligations hereunder
or under law; or (v) any act or failure to act or any alleged act or failure to
act by any Underwriter in connection with, or relating in any manner to, the
Common Stock or the offering contemplated hereby, and which is included as part
of or referred to in any loss, claim, damage, liability or action arising out of
or based upon any matter covered by clause (i) or (ii) above, provided that the
Company shall not be liable under this clause (v) to the extent that a court of
competent jurisdiction shall have determined by a final judgment that such loss,
claim, damage, liability or action resulted directly from any such acts or
failures to act undertaken or omitted to be taken by such Underwriter through
its bad faith or willful misconduct; and to reimburse each Underwriter and each
such controlling person for any and all expenses (including the fees and
disbursements of counsel chosen by NMS) as such expenses are reasonably incurred
by such Underwriter or such controlling person in connection with investigating,
defending, settling, compromising or paying any such loss, claim, damage,
liability, expense or action; provided, however, that the foregoing indemnity
agreement shall not apply to any loss, claim, damage, liability or expense to
the extent, but only to the extent, arising out of or based upon any untrue
statement or alleged untrue statement or omission or alleged omission made in
reliance upon and in conformity with written information furnished to the
Company by the Representatives expressly for use in the Registration Statement,
any preliminary prospectus or the Prospectus (or any amendment or supplement
thereto); and provided, further, that with respect to any preliminary
prospectus, the foregoing indemnity agreement shall not inure to the benefit of
any Underwriter from whom the person asserting any loss, claim, damage,
liability or expense purchased Common Shares, or any person controlling such
Underwriter, if copies of the Prospectus were timely delivered to the
Underwriter pursuant to Section 2 and a copy of the Prospectus (as then amended
or supplemented if the Company shall have furnished any amendments or
supplements thereto) was not sent or given by or on behalf of such Underwriter
to such person, if required by law so to have been delivered, at or prior to the
written confirmation of the sale of the Common Shares to such person, and if the
Prospectus (as so amended or supplemented) would have cured the defect giving
rise to such loss, claim, damage, liability or expense. The indemnity agreement
set forth in this Section 8(a) shall be in addition to any liabilities that the
Company and the Selling Stockholders may otherwise have.
    

   
     (2) Subject to Section 8(e), each of the Selling Stockholders, severally
and not jointly, agrees to indemnify and hold harmless each Underwriter, its
officers and employees, and each person, if any, who controls any Underwriter
within the meaning of the Securities Act and the Exchange Act against any loss,
claim, damage, liability or expense, as incurred, to which such Underwriter or
such controlling person may become subject, under the Securities Act, the
Exchange Act or other federal or state statutory law or regulation, or at common
law or otherwise (including in settlement of any litigation, if such settlement
is effected with the written consent of the Company), insofar as such loss,
claim, damage, liability or expense (or actions in respect thereof as
contemplated below) arises out of or is based (i) upon any untrue statement or
alleged untrue statement of a material fact contained in the Registration
Statement, or any amendment thereto, including any information deemed to be a
part thereof pursuant to Rule 430A or Rule 434 under the Securities Act, or the
omission or alleged omission therefrom of a material fact required to be stated
therein or necessary to make the statements therein not misleading but only to
the extent that such untrue statement or alleged untrue statement or omission or
alleged omission was made in reliance upon and in conformity with information
furnished to the Company or the Underwriters by such Selling Stockholder,
directly or indirectly through such Selling Stockholder's representatives, for
use in the preparation of the Registration Statement or such amendment or
supplement; or (ii) upon any untrue statement or alleged untrue statement of a
material fact contained in any preliminary prospectus or the Prospectus (or any
amendment or supplement thereto), or the omission or alleged omission therefrom
of a material fact necessary in order to make the statements therein, in the
light of the circumstances under which they were made, not misleading but only
to the extent that such untrue statement or alleged untrue statement or omission
or alleged omission was made in reliance upon and in conformity with information
furnished to the Company or the Underwriters by such Selling Stockholder,
directly or through such Selling Stockholders representatives for use in the
preparation of such Prospectus or such amendment or supplement; or (iii) in
whole or in part upon any inaccuracy in the representations and warranties of
such Selling Stockholder contained herein; or (iv) in whole or in part upon any
failure of such Selling Stockholder to perform its obligations hereunder or
under law; or (v) any act or failure to act or any alleged act or failure to act
by any Underwriter in connection with, or relating in any manner to, the Common
Stock or the offering contemplated hereby, and which is included as part of or
referred to in any loss, claim, damage, liability or action arising out of or
based upon any matter covered by clause (i) or (ii) above, provided that such
Selling Stockholder shall not be liable under this clause (v) to the extent that
a court of competent jurisdiction shall have determined by a final judgment that
such loss, claim, damage, liability or action resulted directly from any such
acts or failures to act undertaken or omitted to be taken by such Underwriter
through its bad faith or willful misconduct; and to reimburse each Underwriter
and each such controlling person for any and all expenses (including the fees
and disbursements of counsel chosen my NMS) as such expenses are reasonably
incurred by such Underwriter or such controlling person in connection with
investigating, defending, settling, compromising or paying any such loss, claim,
damage, liability, expense or action; provided, however, that the foregoing
indemnity agreement shall not apply to any loss, claim, damage, liability or
expense to the extent, but only to the extent, arising out of or based upon any
untrue statement or alleged untrue statement or omission or alleged omission
made in reliance upon and in conformity with written information furnished to
the Company by the Representatives expressly for use in the Registration
Statement, any preliminary prospectus or the Prospectus (or any amendment or
supplement thereto); and provided, further, that with respect to any preliminary
prospectus, the foregoing indemnity agreement shall not inure to the benefit of
any Underwriter from whom the person asserting any loss, claim, damage,
liability or expense purchased Common Shares, or any person controlling such
Underwriter, if copies of the Prospectus were timely delivered to the
Underwriter pursuant to Section 2 and a copy of the Prospectus (as then amended
or supplemented if the Company shall have furnished any amendments or
supplements thereto) was not sent or given by or on behalf of such Underwriter
to such person, if required by law so to have been delivered, at or prior to the
written confirmation of the sale of the Common Shares to such person, and if the
Prospectus (as so amended or supplemented) would have cured the defect giving
rise to such loss, claim, damage, liability or expense. The indemnity agreement
set forth in this Section 8(a) shall be in addition to any liabilities that the
Selling Stockholders may otherwise have.
    


                                       18


<PAGE>   20


   
     (b) Indemnification of the Company, its Directors and Officers. Each
Underwriter agrees, severally and not jointly, to indemnify and hold harmless
the Company, each of its directors, each of its officers who signed the
Registration Statement, the Selling Stockholders and each person, if any, who
controls the Company or a Selling Stockholder within the meaning of the
Securities Act or the Exchange Act, against any loss, claim, damage, liability
or expense, as incurred, to which the Company, or any such director, officer,
Selling Stockholder or controlling person may become subject, under the
Securities Act, the Exchange Act, or other federal or state statutory law or
regulation, or at common law or otherwise (including in settlement of any
litigation, if such settlement is effected with the written consent of such
Underwriter), insofar as such loss, claim, damage, liability or expense (or
actions in respect thereof as contemplated below) arises out of or is based upon
any untrue or alleged untrue statement of a material fact contained in the
Registration Statement, any preliminary prospectus or the Prospectus (or any
amendment or supplement thereto), or arises out of or is based upon the omission
or alleged omission to state therein a material fact required to be stated
therein or necessary to make the statements therein not misleading, in each case
to the extent, but only to the extent, that such untrue statement or alleged
untrue statement or omission or alleged omission was made in the Registration
Statement, any preliminary prospectus, the Prospectus (or any amendment or
supplement thereto), in reliance upon and in conformity with written information
furnished to the Company by the Representatives expressly for use therein; and
to reimburse the Company, or any such director, officer, Selling Stockholder or
controlling person for any legal and other expense reasonably incurred by the
Company, or any such director, officer or controlling person in connection with
investigating, defending, settling, compromising or paying any such loss, claim,
damage, liability, expense or action. Each of the Company and the Selling
Stockholders hereby acknowledges that the only information that the Underwriters
have furnished to the Company expressly for use in the Registration Statement,
any preliminary prospectus or the Prospectus (or any amendment or supplement
thereto) are the statements set forth under the caption "Underwriting" in the
Prospectus; and the Underwriters confirm that such statements are correct. The
indemnity agreement set forth in this Section 8(b) shall be in addition to any
liabilities that each Underwriter may otherwise have.
    

     (c) Notifications and Other Indemnification Procedures. Promptly after
receipt by an indemnified party under this Section 8 of notice of the
commencement of any action, such indemnified party will, if a claim in respect
thereof is to be made against an indemnifying party under this Section 8, notify
the indemnifying party in writing of the commencement thereof, but the omission
so to notify the indemnifying party will not relieve it from any liability which
it may have to any indemnified party for contribution or otherwise than under
the indemnity agreement contained in this Section 8 or to the extent it is not
prejudiced as a proximate result of such failure. In case any such action is
brought against any indemnified party and such indemnified party seeks or
intends to seek indemnity from an indemnifying party, the indemnifying party
will be entitled to participate in, and, to the extent that it shall elect,
jointly with all other indemnifying parties similarly notified, by written
notice delivered to the indemnified party promptly after receiving the aforesaid
notice from such indemnified party, to assume the defense thereof with counsel
reasonably satisfactory to such indemnified party; provided, however, if the
defendants in any such action include both the indemnified party and the
indemnifying party and the indemnified party shall have reasonably concluded
that a conflict may arise between the positions of the indemnifying party and
the indemnified party in conducting the defense of any such action or that there
may be legal defenses available to it and/or other indemnified parties which are
different from or additional to those available to the indemnifying party, the
indemnified party or parties shall have the


                                       19


<PAGE>   21


right to select separate counsel to assume such legal defenses and to otherwise
participate in the defense of such action on behalf of such indemnified party or
parties. Upon receipt of notice from the indemnifying party to such indemnified
party of such indemnifying party's election so to assume the defense of such
action and approval by the indemnified party of counsel, the indemnifying party
will not be liable to such indemnified party under this Section 8 for any legal
or other expenses subsequently incurred by such indemnified party in connection
with the defense thereof unless (i) the indemnified party shall have employed
separate counsel in accordance with the proviso in the preceding sentence (it
being understood, however, that the indemnifying party shall not be liable for
the expenses of more than one separate counsel (together with local counsel),
approved by the indemnifying party (NMS in the case of Section 8(b) and Section
9), representing the indemnified parties who are parties to such action) or (ii)
the indemnifying party shall not have employed counsel satisfactory to the
indemnified party to represent the indemnified party within a reasonable time
after notice of commencement of the action, in each of which cases the fees and
expenses of counsel shall be at the expense of the indemnifying party.

     (d) Settlements. The indemnifying party under this Section 8 shall not be
liable for any settlement of any proceeding effected without its written
consent, but if settled with such consent or if there be a final judgment for
the plaintiff, the indemnifying party agrees to indemnify the indemnified party
against any loss, claim, damage, liability or expense by reason of such
settlement or judgment. No indemnifying party shall, without the prior written
consent of the indemnified party, effect any settlement, compromise or consent
to the entry of judgment in any pending or threatened action, suit or proceeding
in respect of which any indemnified party is or could have been a party and
indemnity was or could have been sought hereunder by such indemnified party,
unless such settlement, compromise or consent includes an unconditional release
of such indemnified party from all liability on claims that are the subject
matter of such action, suit or proceeding.

   
     (e) Limitation on Liability of Selling Stockholders. The liability of each
Selling Stockholder under the representations, warranties and agreements
contained herein and under the indemnity and contribution agreements contained
in the provisions of this Section 8 and Section 9 shall be limited to an amount
equal to the aggregate initial public offering price of all Shares sold by such
Selling Stockholder to the Underwriters minus the amount of the underwriting
discount paid thereon to the Underwriters by such Selling Stockholder. The
Company and such Selling Stockholders may agree, as among themselves and without
limiting the rights of the Underwriters under this Agreement, as to the
respective amount of such liability for which they each shall be responsible.
    

Section 9. Contribution.

     If the indemnification provided for in Section 8 is for any reason held to
be unavailable to or otherwise insufficient to hold harmless an indemnified
party in respect of any losses, claims, damages, liabilities or expenses
referred to therein, then each indemnifying party shall contribute to the
aggregate amount paid or payable by such indemnified party, as incurred, as a
result of any losses, claims, damages, liabilities or expenses referred to
therein (i) in such proportion as is appropriate to reflect the relative
benefits received by the Company and the Selling Stockholders, on the one hand,
and the Underwriters, on the other hand, from the offering of the Common Shares
pursuant to this Agreement or (ii) if the allocation provided by clause (i)
above is not permitted by applicable law, in such proportion as is appropriate
to reflect not only the relative benefits referred to in clause (i) above but
also the relative fault of the Company and the Selling Stockholders, on the one
hand, and the Underwriters, on the other hand, in connection with the statements
or omissions or inaccuracies in the representations and warranties herein which
resulted in such losses, claims, damages, liabilities or expenses, as well as
any other relevant equitable considerations. The relative benefits received by
the Company and the Selling Stockholders, on the one hand, and the Underwriters,
on the other hand, in connection with the offering of the Common Shares pursuant
to this Agreement shall be deemed to be in the same respective


                                       20


<PAGE>   22



   
proportions as the total net proceeds from the offering of the Common Shares
pursuant to this Agreement (before deducting expenses) received by the Company
and the Selling Stockholders, and the total underwriting discount received by
the Underwriters, in each case as set forth on the front cover page of the
Prospectus (or, if Rule 434 under the Securities Act is used, the corresponding
location on the Term Sheet) bear to the aggregate initial public offering price
of the Common Shares as set forth on such cover. The relative fault of the
Company and the Selling Stockholders, on the one hand, and the Underwriters, on
the other hand, shall be determined by reference to, among other things, whether
any such untrue or alleged untrue statement of a material fact or omission or
alleged omission to state a material fact or any such inaccurate or alleged
inaccurate representation or warranty relates to information supplied by the
Company, the Selling Stockholders, or the Underwriters, and the parties' 
relative intent, knowledge, access to information and opportunity to correct or 
prevent such statement or omission.
    

   
The amount paid or payable by a party as a result of the losses, claims,
damages, liabilities and expenses referred to above shall be deemed to include,
subject to the limitations set forth in Section 8(c), any legal or other fees or
expenses reasonably incurred by such party in connection with investigating or
defending any action or claim. The provisions set forth in Sections 8(c) and
8(e), with respect to notice of commencement of any action shall apply if a
claim for contribution is to be made under this Section 9; provided, however,
that no additional notice shall be required with respect to any action for which
notice has been given under Section 8(c) for purposes of indemnification.
    

          The Company, the Selling Stockholders and the Underwriters agree that
it would not be just and equitable if contribution pursuant to this Section 9
were determined by pro rata allocation (even if the Underwriters were treated as
one entity for such purpose) or by any other method of allocation which does not
take account of the equitable considerations referred to in this Section 9.

   
          Notwithstanding the provisions of this Section 9, no Underwriter shall
be required to contribute any amount in excess of the underwriting commissions
received by such Underwriter in connection with the Common Shares underwritten
by it and distributed to the public. No person guilty of fraudulent
misrepresentation (within the meaning of Section 11(f) of the Securities Act)
shall be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation. The Underwriters' obligations to contribute
pursuant to this Section 9 are several, and not joint, in proportion to their
respective underwriting commitments as set forth opposite their names in
Schedule A. For purposes of this Section 9, each officer and employee of an
Underwriter and each person, if any, who controls an Underwriter within the
meaning of the Securities Act and the Exchange Act shall have the same rights to
contribution as such Underwriter, and each director of the Company, each officer
of the Company who signed the Registration Statement, each Selling Stockholder
and each person, if any, who controls the Company or a Selling Stockholder with
the meaning of the Securities Act and the Exchange Act shall have the same
rights to contribution as the Company.
    

Section 10. Default of One or More of the Several Underwriters. If, on the First
Closing Date or the Second Closing Date, as the case may be, any one or more of
the several Underwriters shall fail or refuse to purchase Common Shares that it
or they have agreed to purchase hereunder on such date, and the aggregate number
of Common Shares which such defaulting Underwriter or Underwriters agreed but
failed or refused to purchase does not exceed 10% of the aggregate number of the
Common Shares to be purchased on such date, the other Underwriters shall be
obligated, severally, in the proportions that the number of Firm Common Shares
set forth opposite their respective names on Schedule A bears to the aggregate
number of Firm Common Shares set forth opposite the names of all such
non-defaulting


                                       21


<PAGE>   23


Underwriters, or in such other proportions as may be specified by the
Representatives with the consent of the non-defaulting Underwriters, to purchase
the Common Shares which such defaulting Underwriter or Underwriters agreed but
failed or refused to purchase on such date. If, on the First Closing Date or the
Second Closing Date, as the case may be, any one or more of the Underwriters
shall fail or refuse to purchase Common Shares and the aggregate number of
Common Shares with respect to which such default occurs exceeds 10% of the
aggregate number of Common Shares to be purchased on such date, and arrangements
satisfactory to the Representatives and the Company for the purchase of such
Common Shares are not made within 48 hours after such default, this Agreement
shall terminate without liability of any party to any other party except that
the provisions of Section 4, Section 8 and Section 9 shall at all times be
effective and shall survive such termination. In any such case either the
Representatives or the Company shall have the right to postpone the First
Closing Date or the Second Closing Date, as the case may be, but in no event for
longer than seven days in order that the required changes, if any, to the
Registration Statement and the Prospectus or any other documents or arrangements
may be effected.

          As used in this Agreement, the term "Underwriter" shall be deemed to
include any person substituted for a defaulting Underwriter under this Section
10. Any action taken under this Section 10 shall not relieve any defaulting
Underwriter from liability in respect of any default of such Underwriter under
this Agreement.

Section 11. Termination of this Agreement. Prior to the First Closing Date this
Agreement may be terminated by the Representatives by notice given to the
Company if at any time (i) trading or quotation in any of the Company's
securities shall have been suspended or limited by the Commission or by the
Nasdaq Stock Market, or trading in securities generally on either the Nasdaq
Stock Market or the New York Stock Exchange shall have been suspended or
limited, or minimum or maximum prices shall have been generally established on
any of such stock exchanges by the Commission or the NASD; (ii) a general
banking moratorium shall have been declared by any of federal, New York or
California authorities; (iii) there shall have occurred any outbreak or
escalation of national or international hostilities or any crisis or calamity,
or any change in the United States or international financial markets, or any
substantial change or development involving a prospective substantial change in
United States' or international political, financial or economic conditions, as
in the judgment of the Representatives is material and adverse and makes it
impracticable to market the Common Shares in the manner and on the terms
described in the Prospectus or to enforce contracts for the sale of securities;
(iv) in the judgment of the Representatives there shall have occurred any
Material Adverse Change; or (v) the Company shall have sustained a loss by
strike, fire, flood, earthquake, accident or other calamity of such character as
in the reasonable judgment of the Representatives may interfere materially with
the conduct of the business and operations of the Company regardless of whether
or not such loss shall have been insured. Any termination pursuant to this
Section 11 shall be without liability on the part of (a) the Company or the
Selling Stockholders to any Underwriter, except that the Company and the Selling
Stockholders shall be obligated to reimburse the expenses of the Representatives
and the Underwriters pursuant to Sections 4 and 6 hereof, (b) any Underwriter to
the Company or the Selling Stockholders, or (c) of any party hereto to any other
party except that the provisions of Section 8 and Section 9 shall at all times
be effective and shall survive such termination.

Section 12. Representations and Indemnities to Survive Delivery. The respective
indemnities, agreements, representations, warranties and other statements of the
Company, of its officers, of the Selling Stockholders and of the several
Underwriters set forth in or made pursuant to this Agreement will remain in full
force and effect, regardless of any investigation made by or on behalf of any
Underwriter or the Company or any of its or their partners, officers or
directors or any controlling person, or the


                                       22


<PAGE>   24


Selling Stockholders, as the case may be, and will survive delivery of and
payment for the Common Shares sold hereunder and any termination of this
Agreement.

Section 13. All communications hereunder shall be in writing and shall be
mailed, hand delivered or telecopied and confirmed to the parties hereto as
follows:

If to the Representatives:

NationsBanc Montgomery Securities, Inc.
600 Montgomery Street
San Francisco, California 94111
Facsimile:  415-249-5558
Attention:  Richard A. Smith

   with copies to:

NationsBanc Montgomery Securities, Inc.
600 Montgomery Street
San Francisco, California  94111
Facsimile:  (415) 249-5553
Attention:  David A. Baylor, Esq.

         and

Ropes & Gray
One International Place
Boston, MA 02110
Facsimile: (617) 951-7050
Attention: David C. Chapin

If to the Company:

MKS Instruments, Inc.
Six Shattuck Road
Andover, MA 01810
Facsimile:  (978) 975-2350
Attention:  President

         with a copy to:

Hale and Dorr
60 State Street
Boston, MA 02109

Facsimile: (617) 526-5000
Attention: Mark G. Borden

If to the Selling Stockholders:


                                       23


<PAGE>   25


   
BankBoston, N.A.
150 Royall Street
Mall Shop 45-02-62
Canton, MA 02021
Attention: Carole McHugh
    

Section 14. Successors. This Agreement will inure to the benefit of and be
binding upon the parties hereto, including any substitute Underwriters pursuant
to Section 10 hereof, and to the benefit of the employees, officers and
directors and controlling persons referred to in Section 8 and Section 9, and in
each case their respective successors, and no other person will have any right
or obligation hereunder. The term "successors" shall not include any purchaser
of the Common Shares as such from any of the Underwriters merely by reason of
such purchase.

Section 15. Partial Unenforceability. The invalidity or unenforceability of any
Section, paragraph or provision of this Agreement shall not affect the validity
or enforceability of any other Section, paragraph or provision hereof. If any
Section, paragraph or provision of this Agreement is for any reason determined
to be invalid or unenforceable, there shall be deemed to be made such minor
changes (and only such minor changes) as are necessary to make it valid and
enforceable.

Section 16. (a) Governing Law Provisions. THIS AGREEMENT SHALL BE GOVERNED BY
AND CONSTRUED IN ACCORDANCE WITH THE INTERNAL LAWS OF THE STATE OF NEW YORK
APPLICABLE TO AGREEMENTS MADE AND TO BE PERFORMED IN SUCH STATE.

Section 17. Failure of One or More of the Selling Stockholders to Sell and
Deliver Common Shares. If one or more of the Selling Stockholders shall fail to
sell and deliver to the Underwriters the Common Shares to be sold and delivered
by such Selling Stockholders at the First Closing Date pursuant to this
Agreement, then the Underwriters may at their option, by written notice from the
Representatives to the Company and the Selling Stockholders, either (i)
terminate this Agreement without any liability on the part of any Underwriter
or, except as provided in Sections 4, 6, 8 and 9 hereof, the Company or the
Selling Stockholders, or (ii) purchase the shares which the Company and other
Selling Stockholders have agreed to sell and deliver in accordance with the
terms hereof. If one or more of the Selling Stockholders shall fail to sell and
deliver to the Underwriters the Common Shares to be sold and delivered by such
Selling Stockholders pursuant to this Agreement at the First Closing Date, then
the Underwriters shall have the right, by written notice from the
Representatives to the Company and the Selling Stockholders, to postpone the
First Closing Date, but in no event for longer than seven days in order that the
required changes, if any, to the Registration Statement and the Prospectus or
any other documents or arrangements may be effected.

Section 18. General Provisions. This Agreement constitutes the entire agreement
of the parties to this Agreement and supersedes all prior written or oral and
all contemporaneous oral agreements, understandings and negotiations with
respect to the subject matter hereof. This Agreement may be executed in two or
more counterparts, each one of which shall be an original, with the same effect
as if the signatures thereto and hereto were upon the same instrument. This
Agreement may not be amended or modified unless in writing by all of the parties
hereto, and no condition herein (express or implied) may be waived unless waived
in writing by each party whom the condition is meant to benefit. The Table of
Contents and the Section headings herein are for the convenience of the parties
only and shall not affect the construction or interpretation of this Agreement.


                                       24


<PAGE>   26


          Each of the parties hereto acknowledges that it is a sophisticated
business person who was adequately represented by counsel during negotiations
regarding the provisions hereof, including, without limitation, the
indemnification provisions of Section 8 and the contribution provisions of
Section 9, and is fully informed regarding said provisions. Each of the parties
hereto further acknowledges that the provisions of Sections 8 and 9 hereto
fairly allocate the risks in light of the ability of the parties to investigate
the Company, its affairs and its business in order to assure that adequate
disclosure has been made in the Registration Statement, any preliminary
prospectus and the Prospectus (and any amendments and supplements thereto), as
required by the Securities Act and the Exchange Act.

     If the foregoing is in accordance with your understanding of our agreement,
kindly sign and return to the Company the enclosed copies hereof, whereupon this
instrument, along with all counterparts hereof, shall become a binding agreement
in accordance with its terms.


   
    

                                       25


<PAGE>   27
                                Very truly yours,

                                MKS INSTRUMENTS, INC.

                                By:         
                                   --------------------------
   
                                      John R. Bertucci         
                                         President
    

   
                                Selling Stockholders as Listed on the
                                 Attached Schedule B

                                By:          
                                   --------------------------- 
                                        Ronald C. Weigner
    
                                         Attorney-in Fact




                                       26


<PAGE>   28
 
     The foregoing Underwriting Agreement is hereby confirmed and accepted by
the Representatives in San Francisco, California as of the date first above
written.

NATIONSBANC MONTGOMERY SECURITIES LLC
DONALDSON, LUFKIN & JENRETTE SECURITIES CORPORATION
LEHMAN BROTHERS INC.

Acting as Representatives of the 
several Underwriters named in 
the attached Schedule A.

By NATIONSBANC MONTGOMERY SECURITIES LLC


By:________________________
   Title


                                       27


<PAGE>   29


                                   SCHEDULE A

<TABLE>
<CAPTION>

                Underwriters                                        Number of Firm Common Shares to be Purchased
                ------------                                        --------------------------------------------

<S>                                                                 <C> 
NationsBanc Montgomery Securities LLC
Donaldson, Lufkin & Jenrette Securities Corporation
Lehman Brothers Inc.
</TABLE>


                                       26


<PAGE>   30


                                   SCHEDULE B
<TABLE>
<CAPTION>

                                                             Number of Optional Common
                                Number of Firm Common    Shares to be Sold (if Maximum
                                  Shares to be Sold            Option is Exercised)
                                ---------------------    -----------------------------

<S>                                   <C>                          <C>    
The Company                           6,000,000                    975,000

Claire R. Bertucci CBS
Retained Annuity Trust of 1998          50,000                        0

Claire R. Bertucci JCB
Retained Annuity Trust of 1998          50,000                        0

John R. Bertucci CBS Retained
Annuity Trust of 1998                   50,000                        0

John R. Bertucci Family
Retained Annuity Trust of 1998          50,000                        0

John R. Bertucci JCB Retained
Annuity Trust of 1998                   50,000                        0

Claire R. Bertucci CBS
Retained Annuity Trust of 1997          50,000                        0

Claire R. Bertucci JCB
Retained Annuity Trust of 1997          50,000                        0

John R. Bertucci CBS Retained
Annuity Trust of 1997                   50,000                        0

John R. Bertucci Family
Retained Annuity Trust of 1997          50,000                        0

John R. Bertucci JCB Retained
Annuity Trust of 1997                   50,000                        0
</TABLE>


                                       27


<PAGE>   31


                                   EXHIBIT A-1

The final opinion in draft form should be attached as Exhibit A-1 at the time
this Agreement is executed.

          Opinion of counsel for the Company to be delivered pursuant to Section
5(d) of the Underwriting Agreement.

          References to the Prospectus in this Exhibit A include any supplements
thereto at the Closing Date.

          (i) The Company is validly existing as a corporation in good standing
under the laws of The Commonwealth of Massachusetts.

   
          (ii) The authorized, issued and outstanding capital stock of the
Company (including the Common Stock) conform as to legal matters in all material
respects to the descriptions thereof set forth in the Prospectus. All of the
outstanding shares of Common Stock have been duly authorized and validly issued,
are fully paid and nonassessable. The form of certificate used to evidence the
Common Stock is in due and proper form and complies with all applicable
requirements of the charter and by-laws of the Company and The Commonwealth of
Massachusetts.
    

          (iii) The Underwriting Agreement has been duly authorized, executed
and delivered by the Company.

          (iv) The Common Shares to be purchased by the Underwriters from the
Company have been duly authorized for issuance and sale pursuant to the
Underwriting Agreement and, when issued and delivered by the Company pursuant to
the Underwriting Agreement against payment of the consideration set forth
therein, will be validly issued, fully paid and nonassessable.

          (v) Based upon the advice of the staff of the SEC, each of the
Registration Statement and the Rule 462(b) Registration Statement, if any, has
been declared effective by the Commission under the Securities Act. To the
knowledge of such counsel, (A) no stop order suspending the effectiveness of
either of the Registration Statement or the Rule 462(b) Registration Statement,
if any, has been issued under the Securities Act and (B) no proceedings for such
purpose have been instituted or are pending or have been threatened by the
Commission. Any required filing of the Prospectus and any supplement thereto
pursuant to Rule 424(b) under the Securities Act has been made in the manner and
within the time period required by such Rule 424(b).

          (vi) The Registration Statement, including any Rule 462(b)
Registration Statement, the Prospectus, and each amendment or supplement to the
Registration Statement and the Prospectus, as of their respective effective or
issue dates (other than the financial statements and supporting schedules
included therein or in exhibits to or excluded from the Registration Statement,
as to which no opinion need be rendered) comply as to form in all material
respects with the applicable requirements of the Securities Act.


                                       28


<PAGE>   32


          (vii) The Common Shares have been approved for quotation on the Nasdaq
National Market.

   
          (viii) The statements (A) in the Prospectus under the caption
"Shares Eligible for Future Sale" and (B) in Item 14 and Item 15 of the
Registration Statement, insofar as such statements constitute matters of law or
legal conclusions, have been reviewed by such counsel and are correct, in all
material respects.
    

          (ix) To the knowledge of such counsel, there are no legal or
governmental actions, suits or proceedings pending or overtly threatened which
are required to be disclosed in the Registration Statement, other than those
disclosed therein.

          (x) To the knowledge of such counsel, there are no agreements other
than as listed in Exhibit A thereto, required by the Securities Act to be
described to in the Registration Statement or to be filed as exhibits thereto
other than those described to therein or filed or incorporated by reference as
exhibits thereto as is required.

          (xi) No consent, approval, authorization or other order of, or
registration or filing with, any court or other governmental authority or
agency, is required for the Company's execution, delivery and performance of the
Underwriting Agreement and The Company's consummation of the transactions
contemplated thereby and by the Prospectus, except as required under the
Securities Act, applicable state securities or blue sky laws and from the NASD.

   
          (xii) The execution and delivery of the Underwriting Agreement by the
Company and the performance by the Company of its obligations thereunder (other
than performance by the Company of its obligations under the indemnification and
contribution sections of the Underwriting Agreement, as to which no opinion need
be rendered) (A) have been duly authorized by all necessary corporate action on
the part of the Company; (B) will not result in any violation of the provisions
of the charter or by-laws of the Company or any subsidiary; (C) will not
constitute a breach of, or default under, or result in the creation or
imposition of any lien, charge or encumbrance upon any property or assets of the
Company pursuant to any of the agreements listed on Exhibit A thereto filed as
an exhibit to the Registration Statement; or (D) to the knowledge of such
counsel, will not result in any violation of any United States, Federal or
Massachusetts state law, administrative regulation or administrative or court
decree applicable to the Company (other than state securities laws). 
    

          (xiii) The Company is not, and after receipt of payment for the Common
Shares as herein contemplated and the application of the net proceeds therefrom
as described in the Prospectus will not be, an "investment company" within the
meaning of Investment Company Act.

In addition, such counsel shall state that they have participated in conferences
with officers and other representatives of the Company, representatives of the
independent public or certified public accountants for the Company and with
representatives of the Underwriters at which the contents of the Registration
Statement and the Prospectus, and any supplements or amendments thereto, and
related matters were discussed and, while the limitations inherent in the
independent verification of factual matters and the character of determinations
involved in the registration process are such that such counsel is not passing
upon and does not assume any responsibility for the accuracy, completeness or
fairness of the statements contained in the Registration Statement or the
Prospectus (other than as specified above), and any supplements or amendments
thereto, subject to and on the basis of the foregoing, no facts have come to
their attention which has caused them to believe that either the Registration
Statement or any amendments thereto, at the time the Registration Statement or
such amendments became effective, contained an untrue statement of a material
fact or omitted to state a


                                       29


<PAGE>   33


material fact required to be stated therein or necessary to make the statements
therein not misleading or that the Prospectus, as of its date or at the First
Closing Date or the Second Closing Date, as the case may be, contained an untrue
statement of a material fact or omitted to state a material fact necessary in
order to make the statements therein, in the light of the circumstances under
which they were made, not misleading (it being understood that such counsel need
express no belief as to the financial statements or schedules or notes thereto
or other financial or statistical information, or information regarding the
Underwriters or the method of distribution of Common Shares included in the
Registration Statement or the Prospectus or any amendments or supplements
thereto).

   
        In rendering such opinion, such counsel may (A) state that they  render
no opinion as to matters involving the application of laws of any jurisdiction
other than laws of the Commonwealth of Massachusetts or the federal law of the
United States, and (B) rely as to matters of fact, to the extent they deem
reasonable, on certificates of responsible officers of the Company and public
officials.
    


                                       31


<PAGE>   34


                                   EXHIBIT A-2

The final opinion in draft form should be attached as Exhibit A-2 at the time
this Agreement is executed.

          Opinion of counsel for the Company to be delivered pursuant to Section
5(d) of the Underwriting Agreement.

          References to the Prospectus in this Exhibit A include any supplements
thereto at the Closing Date.

          (xiv) The Company has been duly incorporated.

          (xv) The Company has corporate power and authority to own, lease and
operate its properties and to conduct its business as described in the
Prospectus and to enter into and perform its obligations under the Underwriting
Agreement.

          (xvi) The Company is duly qualified as a foreign corporation to
transact business and is in good standing in each United States jurisdiction in
which such qualification is required by reason of the ownership or leasing of
property, except for such jurisdictions where the failure to so qualify or to be
in good standing would not, individually or in the aggregate, result in a
Material Adverse Change.

          (xvii) Each significant subsidiary incorporated (as defined in Rule 
405 under the Securities Act) in the United States has been duly incorporated
and is validly existing as a corporation in good standing under the laws of the
jurisdiction of its incorporation, has corporate power and authority to own,
lease and operate its properties and to conduct its business as described in the
Prospectus and, to the best knowledge of such counsel, is duly qualified as a
foreign corporation to transact business and is in good standing in each United
States jurisdiction in which such qualification is required, whether by reason
of the ownership or leasing of property , except for such jurisdictions where
the failure to so qualify or to be in good standing would not, individually or
in the aggregate, result in a Material Adverse Change.

          (xviii) All of the issued and outstanding capital stock of each such
significant subsidiary incorporated in the United States has been duly
authorized and validly issued, is fully paid and non-assessable and is owned by
the Company, directly or through subsidiaries, to such counsel's knowledge, free
and clear of any security interest, mortgage, pledge, lien, encumbrance or any
pending or threatened claim.

          (xix) No stockholder of the Company or any other person has any
preemptive right, right of first refusal or other similar right to subscribe for
or purchase securities of the Company arising (i) by operation of the charter or
by-laws of the Company or (ii) to the best knowledge of such counsel, otherwise.

          (xx) Except as disclosed in the Prospectus, to the knowledge of such
counsel, there are no persons with registration or other similar rights to have
any equity or debt securities registered for sale under the Registration
Statement or included in the offering contemplated by the Underwriting
Agreement, except for such rights as have been duly waived.


                                       32


<PAGE>   35


          (xxi) To the knowledge of such counsel, the Company is not in
violation of its charter or by-laws, except for such violations as would not,
individually or in the aggregate, result in a Material Adverse Change.

   
          In rendering such opinion, such counsel may rely (A) as to matters
involving the application of laws of any jurisdiction other than the laws of the
Commonwealth of Massachusetts or the federal law of the United States, to the
extent they deem proper and specified in such opinion, upon the opinion (which
shall be dated the First Closing Date or the Second Closing Date, as the case
may be, shall be satisfactory in form and substance to the Underwriters, shall
expressly state that the Underwriters may rely on such opinion as if it were
addressed to them and shall be furnished to the Representatives) of other
counsel of good standing whom they believe to be reliable and who are
satisfactory to counsel for the Underwriters; provided, however, that such
counsel shall further state that they believe that they and the Underwriters are
justified in relying upon such opinion of other counsel, and (B) as to matters
of fact, to the extent they deem proper, on certificates of responsible officers
of the Company and public officials.
    


                                       33


<PAGE>   36


                                   EXHIBIT A-3

The final opinion in draft form should be attached as Exhibit A-3 at the time
this Agreement is executed.

          The opinion of such counsel pursuant to Section 5(i) shall be rendered
to the Representatives at the request of the Company and shall so state therein.
References to the Prospectus in this Exhibit A-3 include any supplements thereto
at the Closing Date.

     (i) The Underwriting Agreement has been duly executed and delivered by or
on behalf of, and is a valid and binding agreement of, such Selling Stockholder,
enforceable in accordance with its terms.

     (ii) The execution and delivery by such Selling Stockholder of, and the
performance by such Selling Stockholder of its obligations under, the
Underwriting Agreement and its Custody Agreement and its Power of Attorney will
not contravene or conflict with, result in a breach of, or constitute a default
under the trust agreement of such Selling Stockholder, or, to the best of such
counsel's knowledge, violate or contravene any provision of applicable law or
regulation, or violate, result in a breach of or constitute a default under the
terms of any other agreement or instrument to which such Selling Stockholder is
a party or by which it is bound, or any judgment, order or decree applicable to
such Selling Stockholder of any court, regulatory body, administrative agency,
governmental body or arbitrator having jurisdiction over such Selling
Stockholder.

     (iii) To the knowledge of such counsel, immediately prior to the date 
hereof, such Selling Stockholder is the sole registered owner of all of the
Common Shares which may be sold by such Selling Stockholder under the
Underwriting Agreement and has the legal right and power, and all authorizations
and approvals required under its trust agreement to enter into the Underwriting
Agreement and its Custody Agreement and its Power of Attorney, to sell, transfer
and deliver all of the Common Shares which may sold by such Selling Stockholder
under the Underwriting Agreement and to comply with its other obligations under
the Underwriting Agreement, its Custody Agreement and its Power of Attorney.

     (iv) Each of the Custody Agreement and Power of Attorney of such Selling
Stockholder has been duly executed and delivered by such Selling Stockholder and
is a valid and binding agreement of such Selling Stockholder, enforceable in
accordance with its terms.

     (v) Assuming that the Underwriters purchase the Common Shares which are
sold by such Selling Stockholder pursuant to the Underwriting Agreement for
value, in good faith and without notice of any adverse claim within the meaning
of Section 8-302 of the Massachusetts Uniform Commercial Code, the delivery of
such Common Shares pursuant to the Underwriting Agreement will, to the knowledge
of such counsel, pass good and valid title to such Common Shares, free and clear
of any security interest, mortgage, pledge, lieu encumbrance or other claim.

     (vi) To the best of such counsel's knowledge, no consent, approval,
authorization or other order of, or registration or filing with, any court or
governmental authority or agency, is required for the consummation by such
Selling Stockholder of the transactions contemplated in the Underwriting
Agreement, except as required under the Securities Act, applicable state
securities or blue sky laws, and from the NASD.


                                       34


<PAGE>   37


          In rendering such opinion, such counsel may rely (A) as to matters
involving the application of laws of any jurisdiction other than the General
Corporation Law of the State of Delaware, the laws of the Commonwealth of
Massachusetts or the federal law of the United States, to the extent they deem
proper and specified in such opinion, upon the opinion (which shall be dated the
First Closing Date or the Second Closing Date, as the case may be, shall be
satisfactory in form and substance to the Underwriters, shall expressly state
that the Underwriters may rely on such opinion as if it were addressed to them
and shall be furnished to the Representative) of other counsel of good standing
whom they believe to be reliable and who are satisfactory to counsel for the
Underwriters; provided, however, that such counsel shall further state that they
believe that they and the Underwriters are justified in relying upon such
opinion of other counsel, and (B) as to matters of fact, to the extent they deem
proper, on certificates of the Selling Stockholders and public officials


                                       35


<PAGE>   38


                                    EXHIBIT B

                                                             ___________, 1999

NationsBanc Montgomery Securities LLC
Donaldson, Lufkin & Jenrette Securities Corporation
Paine Webber

    As Representatives of the Several Underwriters
c/o NationsBanc Montgomery Securities LLC
600 Montgomery Street
San Francisco, California 94111

RE:      MKS Instruments Inc. (the "Company")

Ladies & Gentlemen:

The undersigned is an owner of record or beneficially of certain shares of
Common Stock of the Company ("Common Stock") or securities convertible into or
exchangeable or exercisable for Common Stock. The Company proposes to carry out
a public offering of Common Stock (the "Offering") for which you will act as the
representatives of the underwriters. The undersigned recognizes that the
Offering will be of benefit to the undersigned and will benefit the Company [by,
among other things, raising additional capital for its operations]. The
undersigned acknowledges that you and the other underwriters are relying on the
representations and agreements of the undersigned contained in this letter in
carrying out the Offering and in entering into underwriting arrangements with
the Company with respect to the Offering.

In consideration of the foregoing, the undersigned hereby agrees that the
undersigned will not, without the prior written consent of NMS (which consent
may be withheld in its sole discretion), directly or indirectly, sell, offer,
contract or grant any option to sell (including without limitation any short
sale), pledge, transfer, establish an open "put equivalent position" within the
meaning of Rule 16a-1(h) under the Securities Exchange Act of 1934, or otherwise
dispose of any shares of Common Stock, options or warrants to acquire shares of
Common Stock, or securities exchangeable or exercisable for or convertible into
shares of Common Stock currently or hereafter owned either of record or
beneficially (as defined in Rule 13d-3 under Securities Exchange Act of 1934, as
amended) by the undersigned, or publicly announce the undersigned's intention to
do any of the foregoing, for a period commencing on the date hereof and
continuing through the close of trading on the date 180 days after the date of
the Prospectus. The undersigned also agrees and consents to the entry of stop
transfer instructions with the Company's transfer agent and registrar against
the transfer of shares of Common Stock or securities convertible into or
exchangeable or exercisable for Common Stock held by the undersigned except in
compliance with the foregoing restrictions.

With respect to the Offering only, the undersigned waives any registration
rights relating to registration under the Securities Act of any Common Stock
owned either of record or beneficially by the undersigned, including any rights
to receive notice of the Offering.

This agreement is irrevocable and will be binding on the undersigned and the
respective successors, heirs, personal representatives, and assigns of the
undersigned.


                                        1


<PAGE>   39


Printed Name of Holder

By:
    Signature

Printed Name of Person Signing 
(and indicate capacity of person signing if
signing as custodian, trustee, or on behalf 
of an entity)


                                        1



<PAGE>   1
                                                                     Exhibit 3.2
   
                                                          FEDERAL IDENTIFICATION
                                                          No. 04-2277512
    

   
    

                        THE COMMONWEALTH OF MASSACHUSETTS
   
                             William Francis Galvin
                         Secretary of the Commonwealth
             One Ashburton Place, Boston, Massachusetts 02108-1512
                       RESTATED ARTICLES OF ORGANIZATION
                     (GENERAL LAWS, CHAPTER 156B, SECTION 74)
    


                                ----------------

   
         We, John R. Bertucci                         , President and
                  Richard S. Chute                            , Clerk
    
   
of                MKS Instruments, Inc.
   ----------------------------------------------------------------------------
                          (Exact name of corporation)
    

   
located at SIX SHATTUCK ROAD, ANDOVER, MASSACHUSETTS 01810 do hereby certify
that the following Restatement of the Articles of Organization was duly adopted
at a meeting held on February 17, 1999, by vote of the directors/or:
    
   
    
   
5,177,940 shares of CLASS A COMMON  out of 5,177,940 shares outstanding,
_________           _______________        _________
                    (type, class &
                    series, if any) 
    

   
6,857,500 shares of CLASS B COMMON out of 6,857,501 shares outstanding, and
_________           ______________        _________
                   (type, class &
                   series, if any) 
    
   
_________ shares of _______________ out of _________ shares outstanding,
                   (type, class &
                    series, if any) 
    

   
being at least two-thirds of each type, class or series of stock outstanding and
entitled to vote thereon and of each type class or series of stock whose rights
are adversely affected thereby:
    

   
                                   ARTICLE I

             The name of the corporation is:
    

                  MKS Instruments, Inc.

   
                                   ARTICLE II

             The purpose of the corporation is to engage in the following 
             business activities :
    

C   [ ]           See Continuation Sheets 2A and 2B.

P   [ ] 

M   [ ]

RA  [ ]

   
       Note:   If the space provided under any article or item on this form is
               insufficient, additions shall be set forth on separate 8 1/2 x 11
               sheets of paper with a left hand margin of at least 1 inch. 
               Additions to more than one article may be made on a single sheet
               so long as each article requiring each such addition is clearly
               indicated.
    


<PAGE>   2



                              CONTINUATION SHEET 2A

2.       THE PURPOSES FOR WHICH THE CORPORATION IS FORMED ARE AS FOLLOWS:

         To design, manufacture, sell, lease and license instruments of all
kinds, including electromechanical, electronic and mechanical gauges for the
measurement of pressure, temperature, acceleration, flow and level of liquids
and gases; to design, manufacture, sell, lease and license control systems
incorporating measuring devices, and control systems separate from measuring
devices, for the control of production processes and operations of all kinds; to
design, manufacture, sell, lease and license instrumentation for military use;
to design, manufacture, sell, lease and license instrumentation for use in
research laboratories, in industry, in educational institutions, for medical
purposes and for use elsewhere and for other purposes; and in general to design,
manufacture, sell, lease and license electro-mechanical, electronic and
mechanical devices of all kinds.

         To buy and sell at wholesale and retail, or otherwise, to manufacture,
produce, adapt, repair, dispose of, export, import and in any other manner to
deal in goods, wares, merchandise, articles and things of manufacture or
otherwise of all materials, supplies and other articles and things necessary or
convenient for use in connection with any of said businesses or any other
business or any part thereof; and to manufacture, repair, purchase, sell, lease,
dispose of and otherwise deal in machinery, tools, and appliances which are or
may be used in connection with the purchase, sale, production, adaption, repair,
disposition of, export, import or other dealings in said goods, wares,
merchandise, articles and things.

         To purchase, lease or otherwise acquire as a going concern or otherwise
all or any part of the franchises, rights, property, assets, business, good will
or capital stock of any persons, firm, corporation, trust or association engaged
in whole or in part in any business in which this corporation is empowered to
engage, or in any other business; to pay for the same in whole or in part in
cash, stock, bonds, notes, securities or other evidence of indebtedness of this
corporation or in any other manner; to assume as part of the consideration or
otherwise any and all debts, contracts or liabilities, matured or unmatured,
fixed or contingent, of any such person, firm or corporation, trust or
association; and to operate, manage, develop and generally to carry on the whole
or any part of any such business under any name or names which it may select or
designate.




<PAGE>   3



                              CONTINUATION SHEET 2B

         To construct, lease, hire, purchase or otherwise acquire and hold or
maintain, and to rebuild, enlarge, improve, furnish, equip, alter, operate and
dispose of warehouses, factories, offices and other buildings, real estate,
structures or parts thereof, and appliances for the preparation, manufacture,
purchase, sale and distribution of goods, wares, merchandise, things, and
articles of all kinds.

         To acquire, hold, use, sell, assign, lease, grant licenses in respect
of, mortgage, or otherwise dispose of franchises, letters patent of the United
States or of any foreign country, patent rights, licenses and privileges,
inventions, improvements and processes, systems, copyrights, trade-marks and
trade names, relating to, or useful in connection with, any business of this
corporation.

         To buy or otherwise acquire, to sell, assign, pledge, or otherwise
dispose of and deal in stocks, bonds, securities, notes and other obligations of
any person, firm or corporation, including this corporation, organized for or
engaged in similar or cognate purposes; also stocks, bonds, securities, notes,
and other obligations of any person, firm, or corporation, including this
corporation, which it may be found or deemed necessary, valuable, or convenient
for this corporation to acquire and deal in, in pursuance or furtherance of or
in connection with the businesses herein specified, or any other business.

         To borrow money and contract indebtedness for all proper corporate
purposes, to issue bonds, notes, and other evidences of indebtedness, to secure
the same by pledge, mortgage, or lien on all or any part of the property of the
corporation, tangible or intangible; and to assume or guarantee or secure in
like manner or otherwise, the leases, contracts, or other obligations, fixed or
contingent, or the payment of any dividends on any stock or shares or of the
principal or interest on any bonds, notes, or other evidences of indebtedness of
any person, firm, corporation, trust, or association in which this corporation
has a financial interest.

         To enter into, make, and perform contracts of every name, nature, and
kind with any person, firm, association, or corporation which may be deemed
valuable, expedient, or convenient for this corporation in pursuance of or in
furtherance of or in connection with any of the objects of incorporation of this
corporation or in connection with any of the businesses or purposes herein
specified.

         The enumeration of specific powers herein shall not be construed as
limiting or restricting in any way the general powers herein set forth, but
nothing herein contained shall be construed as authorizing the business of
banking.







<PAGE>   4
                                  ARTICLE III

State the total number of shares and par value, if any, of each class of stock
which the corporation is authorized to issue:

   
<TABLE>
<CAPTION>
               WITHOUT PAR VALUE                             WITH PAR VALUE
               -----------------                    -----------------------------

    TYPE        NUMBER OF SHARES        TYPE        NUMBER OF SHARES   PAR VALUE
- ------------    ----------------    ------------    ----------------   ---------
<S>             <C>                 <C>              <C>                <C>
Common             50,000,000       Common
Preferred                           Preferred           2,000,000        $0.01
</TABLE>
    


                                   ARTICLE IV

If more than one  class of stock is authorized, state a distinguishing 
designation for each class. Prior to the issuance of any shares of a class, if 
shares of another class are outstanding, the corporation must provide a 
description of the preferences, voting powers, qualifications, and special and 
relative rights or privileges of that class and of each other class of which 
shares are outstanding and of each series then established within any class.

                      See Continuation Sheets 4A, 4B and 4C

                                   ARTICLE V

The restrictions, if any, imposed by the Articles of Organization upon the
transfer of shares of stock of any class are:

                      None


                                   ARTICLE VI

**Other lawful provisions, if any, for the conduct and regulation of the
business and affairs of the corporation, for its voluntary dissolution, or for
limiting, defining, or regulating the powers of the corporation, or of its
directors or stockholders, or of any class of stockholders:

                      See Continuation Sheets 6A, 6B, 6C, 6D, 6E, 6F and 6G




**If there are no provisions state "None".

   
Note: the preceding six (6) articles are considered to be permanent and may 
ONLY be changed by filing appropriate Articles of Amendment.
    


<PAGE>   5





                              CONTINUATION SHEET 4A

         The total number of shares of all classes of stock which the
corporation shall have authority to issue is 52,000,000 shares, consisting of
(i) 50,000,000 shares of Common Stock, no par value per share ("Common Stock"),
and (ii) 2,000,000 shares of Preferred Stock, $0.01 par value per share
("Preferred Stock"). Upon the filing of the corporation's Restated Articles of
Organization on March __, 1999 (the "Mandatory Conversion Date") each share of
Class A Common Stock, no par value per share, and each share of Class B Common
Stock, no par value per share (together with the Class A Common Stock, the
"Class Common Stock"), shall be converted into one share of Common Stock, no par
value per share. All holders of record of shares of Class Common Stock shall be
given written notice of the Mandatory Conversion Date and the place designated
for mandatory conversion of all such shares of Class Common Stock pursuant to
this provision. Such notice need not be given in advance of the occurrence of
the Mandatory Conversion Date. Such notice shall be sent by first class or
registered mail, postage prepaid, to each record holder of Class Common Stock at
such holder's address last shown on the records of the transfer agent for the
Class Common Stock (or the records of the corporation, if it serves as its own
transfer agent). Upon receipt of such notice, each holder of shares of Class
Common Stock shall surrender his or its certificate or certificates for all such
shares to the corporation at the place designated in such notice, and shall
thereafter receive certificates for the number of shares of Common Stock equal
to the number of shares of Class Common Stock represented by such certificates.
On the Mandatory Conversion Date, all rights with respect to the Class Common
Stock so converted, including the rights, if any, to receive notices and vote
(other than as a holder of Common Stock) will terminate, except only the rights
of the holders thereof, upon surrender of their certificate or certificates
therefor, to receive certificates for the number of shares of Common Stock into
which such Class Common Stock has been converted, and payment of any declared
but unpaid dividends thereon. If so required by the corporation, certificates
surrendered for conversion shall be endorsed or accompanied by written
instrument or instruments of transfer, in form satisfactory to the corporation,
duly executed by the registered holder or by his or its attorney duly authorized
in writing. As soon as practicable after the Mandatory Conversion Date and the
surrender of the certificate or certificates for Class Common Stock, the
corporation shall cause to be issued and delivered to such holder, or on his or
its written order, a certificate or certificates for the number of full shares
of Common Stock issuable on such conversion in accordance with the provisions
hereof.

         All certificates evidencing shares of Class Common Stock which are
required to be surrendered for conversion in accordance with the provisions
hereof shall, from and after the Mandatory Conversion Date, be deemed to have
been retired and cancelled and the shares of Class Common Stock represented
thereby converted into Common Stock for all purposes, notwithstanding the
failure of the holder or holders thereof to surrender such certificates on or
prior to such date.



<PAGE>   6



                              CONTINUATION SHEET 4B

         The following is a statement of the designation and the powers,
privileges and rights, and the qualifications, limitations or restrictions
thereof in respect of each class of capital stock of the corporation.

A.       COMMON STOCK.

         1. GENERAL. The voting, dividend and liquidation rights of the holders
of the Common Stock are subject to and qualified by the rights of the holders of
the Preferred Stock of any series as may be designated by the Board of Directors
upon any issuance of the Preferred Stock of any series.

         2. VOTING. The holders of the Common Stock are entitled to one vote for
each share held at all meetings of stockholders (and written actions in lieu of
meetings). There shall be no cumulative voting.

         3. DIVIDENDS. Dividends may be declared and paid on the Common Stock
from funds lawfully available therefor as and when determined by the Board of
Directors and subject to any preferential dividend rights of any then
outstanding Preferred Stock.

         4. LIQUIDATION. Upon the dissolution or liquidation of the corporation,
whether voluntary or involuntary, holders of Common Stock will be entitled to
receive all assets of the corporation available for distribution to its
stockholders, subject to any preferential rights of any then outstanding
Preferred Stock.

B.       PREFERRED STOCK.

         Preferred Stock may be issued from time to time in one or more series,
each of such series to have such terms as stated or expressed herein and in the
resolution or resolutions providing for the issue of such series adopted by the
Board of Directors of the corporation as hereinafter provided. Any shares of
Preferred Stock which may be redeemed, purchased or acquired by the corporation
may be reissued except as otherwise provided by law. Different series of
Preferred Stock shall not be construed to constitute different classes of shares
for the purposes of voting by classes unless expressly provided.


<PAGE>   7



                              CONTINUATION SHEET 4C

         Authority is hereby expressly granted to the Board of Directors from
time to time to issue the Preferred Stock in one or more series, and in
connection with the creation of any such series, by resolution or resolutions
providing for the issue of the shares thereof, to determine and fix such voting
powers, full or limited, or no voting powers, and such designations, preferences
and relative participating, optional or other special rights, and
qualifications, limitations or restrictions thereof, including without
limitation thereof, dividend rights, conversion rights, redemption privileges
and liquidation preferences, as shall be stated and expressed in such
resolutions, all to the full extent now or hereafter permitted by Chapter 156B
of the Massachusetts General Laws. Without limiting the generality of the
foregoing, the resolutions providing for issuance of any series of Preferred
Stock may provide that such series shall be superior or rank equally or be
junior to the Preferred Stock of any other series to the extent permitted by
law. No vote of the holders of the Preferred Stock or Common Stock shall be a
prerequisite to the issuance of any shares of any series of the Preferred Stock
authorized by and complying with the conditions of the Articles of Organization,
the right to have such vote being expressly waived by all present and future
holders of the capital stock of the corporation.



<PAGE>   8



                              CONTINUATION SHEET 6A


6A.      LIMITATION OF DIRECTOR LIABILITY

         Except to the extent that Chapter 156B of the Massachusetts General
Laws prohibits the elimination or limitation of liability of directors for
breaches of fiduciary duty, no director of the corporation shall be personally
liable to the corporation or its stockholders for monetary damages for any
breach of fiduciary duty as a director, notwithstanding any provision of law
imposing such liability. No amendment to or repeal of this provision shall apply
to or have any effect on the liability or alleged liability of any director of
the corporation for or with respect to any acts or omissions of such director
occurring prior to such amendment.


6B.      INDEMNIFICATION

         1. ACTIONS, SUITS AND PROCEEDINGS. The corporation shall indemnify each
person who was or is a party or is threatened to be made a party to any
threatened, pending or completed action, suit or proceeding, whether civil,
criminal, administrative or investigative, by reason of the fact that he is or
was, or has agreed to become, a director or officer of the corporation, or is or
was serving, or has agreed to serve, at the request of the corporation, as a
director or officer of, or in a similar capacity with, another organization or
in any capacity with respect to any employee benefit plan of the corporation
(all such persons being referred to hereafter as an "Indemnitee"), or by reason
of any action alleged to have been taken or omitted in such capacity, against
all expenses (including attorneys' fees), judgments and fines incurred by him or
on his behalf in connection with such action, suit or proceeding and any appeal
therefrom, unless the Indemnitee shall be finally adjudicated in such action,
suit or proceeding not to have acted in good faith in the reasonable belief that
his action was in the best interests of the corporation or, to the extent such
matter relates to service with respect to an employee benefit plan, in the best
interests of the participants or beneficiaries of such employee benefit plan.
Notwithstanding anything to the contrary in this Article, except as set forth in
Section 6 below, the corporation shall not indemnify an Indemnitee seeking
indemnification in connection with a proceeding (or part thereof) initiated by
the Indemnitee unless the initiation thereof was approved by the Board of
Directors of the












<PAGE>   9



                              CONTINUATION SHEET 6B


corporation. Notwithstanding anything to the contrary in this Article, the
corporation shall not indemnify an Indemnitee to the extent such Indemnitee is
reimbursed from the proceeds of insurance, and in the event the corporation
makes any indemnification payments to an Indemnitee and the Indemnitee is
subsequently reimbursed from the proceeds of insurance, such Indemnitee shall
promptly refund such indemnification payments to the corporation to the extent
of such insurance reimbursement.

         2. SETTLEMENTS AND COMPROMISE. The right to indemnification conferred
in this Article shall include the right to be paid by the corporation for
amounts paid in settlement or compromise of any such action, suit or proceeding
and any appeal therefrom, and all expenses (including attorneys' fees) incurred
in connection with such settlement or compromise, pursuant to a consent decree
or otherwise, unless and to the extent it is determined pursuant to Section 5
below that the Indemnitee did not act in good faith in the reasonable belief
that his action was in the best interests of the corporation or, to the extent
such matter relates to service with respect to an employee benefit plan, in the
best interests of the participants or beneficiaries of such employee benefit
plan.

         3. NOTIFICATION AND DEFENSE OF CLAIM. As a condition precedent to his
right to be indemnified, the Indemnitee must notify the corporation in writing
as soon as practicable of any action, suit, proceeding or investigation
involving him for which indemnity will or could be sought. With respect to any
action, suit, proceeding or investigation of which the corporation is so
notified, the corporation will be entitled to participate therein at its own
expense and/or to assume the defense thereof at its own expense, with legal
counsel reasonably acceptable to the Indemnitee. After notice from the
corporation to the Indemnitee of its election so to assume such defense, the
corporation shall not be liable to the Indemnitee for any legal or other
expenses subsequently incurred by the Indemnitee in connection with such claim,
other than as provided below in this Section 3. The Indemnitee shall have the
right to employ his own counsel in connection with such claim, but the fees and
expenses of such counsel incurred after notice from the corporation of its
assumption of the defense thereof shall be at the expense of the Indemnitee
unless (i) the employment of counsel by the Indemnitee has been authorized by
the corporation, (ii) counsel to the Indemnitee shall have reasonably concluded
that there may be a conflict of interest or position on any significant issue





<PAGE>   10



                              CONTINUATION SHEET 6C


between the corporation and the Indemnitee in the conduct of the defense of such
action or (iii) the corporation shall not in fact have employed counsel to
assume the defense of such action, in each of which cases the fees and expenses
of counsel for the Indemnitee shall be at the expense of the corporation, except
as otherwise expressly provided by this Article. The corporation shall not be
entitled, without the consent of the Indemnitee, to assume the defense of any
claim brought by or in the right of the corporation or as to which counsel for
the Indemnitee shall have reasonably made the conclusion provided for in clause
(ii) above.

         4. ADVANCE OF EXPENSES. Subject to the provisions of Section 5 below,
in the event that the corporation does not assume the defense pursuant to
Section 3 of this Article of any action, suit, proceeding or investigation of
which the corporation receives notice under this Article, any expenses
(including attorneys' fees) incurred by an Indemnitee in defending a civil or
criminal action, suit, proceeding or investigation or any appeal therefrom shall
be paid by the corporation in advance of the final disposition of such matter;
PROVIDED, HOWEVER, that the payment of such expenses incurred by an Indemnitee
in advance of the final disposition of such matter shall be made only upon
receipt of an undertaking by or on behalf of the Indemnitee to repay all amounts
so advanced in the event that it shall ultimately be determined that the
Indemnitee is not entitled to be indemnified by the corporation as authorized in
this Article. Such undertaking shall be accepted without reference to the
financial ability of the Indemnitee to make such repayment.

         5. PROCEDURE FOR INDEMNIFICATION. In order to obtain indemnification or
advancement of expenses pursuant to Section 1, 2 or 4 of this Article, the
Indemnitee shall submit to the corporation a written request, including in such
request such documentation and information as is reasonably available to the
Indemnitee and is reasonably necessary to determine whether and to what extent
the Indemnitee is entitled to indemnification or advancement of expenses. Any
such indemnification or advancement of expenses shall be made promptly, and in
any event within 60 days after receipt by the corporation of the written request
of the Indemnitee, unless the corporation determines within such 60-day period
that the Indemnitee did not meet the applicable standard of conduct set forth in
Section 1 or 2, as the case may be. Such determination shall be made in each
instance by (a) a majority vote of a quorum of the directors of the corporation,
(b) a majority vote of a


<PAGE>   11



                              CONTINUATION SHEET 6D


quorum of the outstanding shares of stock of all classes entitled to vote for
directors, voting as a single class, which quorum shall consist of stockholders
who are not at that time parties to the action, suit or proceeding in question,
(c) independent legal counsel (who may, to the extent permitted by law, be
regular legal counsel to the corporation), or (d) a court of competent
jurisdiction.

         6. REMEDIES. The right to indemnification or advances as granted by
this Article shall be enforceable by the Indemnitee in any court of competent
jurisdiction if the corporation denies such request, in whole or in part, or if
no disposition thereof is made within the 60-day period referred to above in
Section 5. Unless otherwise required by law, the burden of proving that the
Indemnitee is not entitled to indemnification or advancement of expenses under
this Article shall be on the corporation. Neither the failure of the corporation
to have made a determination prior to the commencement of such action that
indemnification is proper in the circumstances because the Indemnitee has met
the applicable standard of conduct, nor an actual determination by the
corporation pursuant to Section 5 that the Indemnitee has not met such
applicable standard of conduct, shall be a defense to the action or create a
presumption that the Indemnitee has not met the applicable standard of conduct.
The Indemnitee's expenses (including attorneys' fees) incurred in connection
with successfully establishing his right to indemnification, in whole or in
part, in any such proceeding shall also be indemnified by the corporation.

         7. SUBSEQUENT AMENDMENT. No amendment, termination or repeal of this
Article or of the relevant provisions of Chapter 156B of the Massachusetts
General Laws or any other applicable laws shall affect or diminish in any way
the rights of any Indemnitee to indemnification under the provisions hereof with
respect to any action, suit, proceeding or investigation arising out of or
relating to any actions, transactions or facts occurring prior to the final
adoption of such amendment, termination or repeal.

         8. OTHER RIGHTS. The indemnification and advancement of expenses
provided by this Article shall not be deemed exclusive of any other rights to
which an Indemnitee seeking indemnification or advancement of expenses may be
entitled under any law (common or statutory), agreement or vote of stockholders
or directors or otherwise, both


<PAGE>   12



                              CONTINUATION SHEET 6E


as to action in his official capacity and as to action in any other capacity
while holding office for the corporation, and shall continue as to an Indemnitee
who has ceased to be a director or officer, and shall inure to the benefit of
the estate, heirs, executors and administrators of the Indemnitee. Nothing
contained in this Article shall be deemed to prohibit, and the corporation is
specifically authorized to enter into, agreement with officers and directors
providing indemnification rights and procedures different from those set forth
in this Article. In addition, the corporation may, to the extent authorized from
time to time by its Board of Directors, grant indemnification rights to other
employees or agents of the corporation or other persons serving the corporation
and such rights may be equivalent to, or greater or less than, those set forth
in this Article.

         9. PARTIAL INDEMNIFICATION. If an Indemnitee is entitled under any
provision of this Article to indemnification by the corporation for some or a
portion of the expenses (including attorneys' fees), judgments, fines or amounts
paid in settlement or compromise actually and reasonably incurred by him or on
his behalf in connection with any action, suit, proceeding or investigation and
any appeal therefrom but not, however, for the total amount thereof, the
corporation shall nevertheless indemnify the Indemnitee for the portion of such
expenses (including attorneys' fees), judgments, fines or amounts paid in
settlement or compromise to which the Indemnitee is entitled.

         10. INSURANCE. The corporation may purchase and maintain insurance, at
its expense, to protect itself and any director, officer, employee or agent of
the corporation or another organization or employee benefit plan against any
expense, liability or loss incurred by him in any such capacity, or arising out
of his status as such, whether or not the corporation would have the power to
indemnify such person against such expense, liability or loss under Chapter 156B
of the Massachusetts General Laws.

         11. MERGER OR CONSOLIDATION. If the corporation is merged into or
consolidated with another corporation and the corporation is not the surviving
corporation, the surviving corporation shall assume the obligations of the
corporation under this Article with respect to any action, suit, proceeding or
investigation arising out of or relating to any actions, transactions or facts
occurring prior to the date of such merger or consolidation.




<PAGE>   13



                              CONTINUATION SHEET 6F


         12. SAVINGS CLAUSE. If this Article or any portion hereof shall be
invalidated on any ground by any court of competent jurisdiction, then the
corporation shall nevertheless indemnify each Indemnitee as to any expenses
(including attorneys' fees), judgments, fines and amounts paid in settlement or
compromise in connection with any action, suit, proceeding or investigation,
whether civil, criminal or administrative, including an action by or in the
right of the corporation, to the fullest extent permitted by any applicable
portion of this Article that shall not have been invalidated and to the fullest
extent permitted by applicable law.

         13. SUBSEQUENT LEGISLATION. If the Massachusetts General Laws are
amended after adoption of this Article to expand further the indemnification
permitted to Indemnitees, then the corporation shall indemnify such persons to
the fullest extent permitted by the Massachusetts General Laws, as so amended.

6C.      OTHER PROVISIONS

         (a) The directors may make, amend, or repeal the By-Laws in whole or in
part, except with respect to any provision of such By-Laws which by law or these
Articles of Organization or the By-Laws requires action by the stockholders.

         (b) Meetings of the stockholders of the corporation may be held
anywhere in the United States.

         (c) The corporation shall have the power to be a partner in any
business enterprise which this corporation would have the power to conduct by
itself.

         (d) The corporation, by vote of at least sixty-six and two-thirds
percent (66 2/3%) of the stock outstanding and entitled to vote thereon (or if
there are two or more classes of stock entitled to vote as separate classes,
then by vote of at least sixty-six and two-thirds percent (66 2/3%) of each such
class of stock outstanding), may (i) authorize any amendment to its Articles of
Organization pursuant to Section 71 of Chapter 156B of the Massachusetts General
Laws, as amended from time to time, (ii) authorize the sale, lease or exchange
of all or substantially all of its property and assets, including its goodwill,
pursuant to Section 75 of Chapter 156B of the Massachusetts


<PAGE>   14



                              CONTINUATION SHEET 6G


General Laws, as amended from time to time, and (iii) approve an agreement of
merger or consolidation pursuant to Section 78 of Chapter 156B of the
Massachusetts General Laws, as amended from time to time; PROVIDED, however,
that if any such (i) amendment to its Articles of Organization, (ii) sale,
lease, or exchange or (iii) merger or consolidation (each as more fully
described above) has been approved by a majority of the Board of Directors of
the corporation, then the corporation may authorize or approve such action by
vote of a majority of the stock outstanding and entitled to vote thereon (or if
there are two or more classes of stock entitled to vote as separate classes,
then by vote of a majority of each such class outstanding).

         (e) Chapter 110F of the Massachusetts General Laws, as it may be
amended from time to time, shall not apply to the corporation.



<PAGE>   15
   
                                  ARTICLE VII
    

   
The effective date of the restated Articles of Organization of the corporation 
shall be the date approved and filed by the Secretary of the Commonwealth. If a 
later effective date is desired, specify such date which shall not be more 
than thirty days after the date of filing.
    

   
                                  ARTICLE VIII
    

   
The information contained in Article VIII is not a permanent part of the 
Articles of Organization.
    

   
a.    The street address (post office box are not acceptable) of the principle
office of the corporation in Massachusetts is:
    
     
   
      6 Shattuck Road, Andover, MA 01810
    

   
b.    The name, residential address and post office address of each director and
officer of the corporation is as follows. 
    

   
<TABLE>
<CAPTION>
            NAME              RESIDENTIAL ADDRESS        POST OFFICE ADDRESS
<S>                           <C>                      <C>

President:  John R. Bertucci                           c/o MKS Instruments, Inc.
                                                       6 Shattuck Road
                                                       Andover, MA  01810

Treasurer:  William Donlan                             c/o MKS Instruments, Inc.
                                                       6 Shattuck Road
                                                       Andover, MA  01810        

Clerk:      Richard S. Chute                           c/o MKS Instruments, Inc.
                                                       6 Shattuck Road
                                                       Andover, MA  01810

Directors:  John R. Bertucci                           c/o MKS Instruments, Inc.
                                                       6 Shattuck Road
                                                       Andover, MA  01810  
            Richard S. Chute                           c/o MKS Instruments, Inc.
                                                       6 Shattuck Road
                                                       Andover, MA  01810
            Owen W. Robbins                            c/o MKS Instruments, Inc.
                                                       6 Shattuck Road
                                                       Andover, MA  01810
            Robert J. Therrien                         c/o MKS Instruments, Inc.
                                                       6 Shattuck Road
                                                       Andover, MA  01810
            Louis P. Valente                           c/o MKS Instruments, Inc.
                                                       6 Shattuck Road
                                                       Andover, MA  01810
</TABLE>
    

   
c.    The fiscal year (i.e., tax year) of the corporation shall end on the last 
day of the month of:

          December

d.    The name and business address of the resident agent, if any, of the 
corporation is:      
    

   
**We further certify that the foregoing Restated Articles of Organization affect
no amendments to the Articles of Organization of the corporation as heretofore
amended, except amendments to the following articles. Briefly describe 
amendments below:
    


   
                            See Continuation Sheet 8
    

   
SIGNED UNDER THE PENALTIES OF PERJURY, this ________________________ day of 
___________________, 1999.




_____________________________________________________, President

_____________________________________________________, Clerk


 *Delete the inapplicable words

**If there are no amendments, state "None."
    
<PAGE>   16

   
                              Continuation Sheet 8



Article 3.        Is amended to: (i) eliminate authorized shares of Class A
                  Common Stock, no par value per share ("Class A Common Stock"),
                  and Class B Common Stock, no par value per share ("Class B
                  Common Stock"); (ii) increase the authorized number of shares
                  of Common Stock, no par value per share ("Common Stock") to
                  50,000,000 shares and; (iii) authorize issuance of up to
                  2,000,000 shares of Preferred Stock.

Article 4.        Is amended to: (i) delete any and all provisions describing or
                  relating to rights and preferences of Class A Common Stock and
                  Class B Common Stock; (ii) provide that each outstanding
                  share of Class A Common Stock and Class B Common Stock has
                  been converted into one share of Common Stock; (iii) provide a
                  statement of the designation and the powers, privileges and
                  rights, and the qualification, limitations or restrictions
                  thereof in respect of each class of capital stock of the
                  corporation; and (iv) authorize the Board of Directors to
                  issue Preferred Stock in one or more series and create any
                  such series of Preferred Stock without requiring a vote of the
                  holders of Preferred Stock or Common Stock as a prerequisite
                  to the issuance of any shares of any such Preferred Stock and
                  restate Article 4 in its entirety.

Article 6.        Is amended to (i) delete the provision relating to limitation
                  of director liability and replace it with Section 6A,
                  "Limitation of Director Liability;" (ii) add Section 6B
                  "Indemnification;" and (iii) add Section 6C "Other
                  Provisions."

    
<PAGE>   17
   
                       THE COMMONWEALTH OF MASSACHUSETTS

                       RESTATED ARTICLES OF ORGANIZATION
                    (General Laws, Chapter 156B, Section 74)


                 _____________________________________________

                 I hereby approve the within Restated Articles
                 of Organization and, the filing fee in the
                 amount of $_______ having been paid, said
                 articles are deemed to have been filed with me
                 this ______ day of _____________, 19__.

                 Effective date:_______________________________




                             WILLIAM FRANCIS GALVIN
                         Secretary of the Commonwealth




                         TO BE FILLED IN BY CORPORATION
                      Photocopy of document to be sent to:


                       Emma R. Petty, Corporate Paralegal
               ------------------------------------------------
                       Hale and Dorr LLP
                       60 State Street
               ------------------------------------------------

                       Boston, MA 02109
               ------------------------------------------------

               Telephone:          617-526-6000
                         --------------------------------------
    

<PAGE>   1
                                                                   EXHIBIT 10.24

          TAX INDEMNIFICATION AND S CORPORATION DISTRIBUTION AGREEMENT


     This TAX INDEMNIFICATION AND S CORPORATION DISTRIBUTION AGREEMENT (the
"Agreement") is entered into as of March ___, 1999 between MKS INSTRUMENTS,
INC., a Massachusetts corporation (the "Company"), and the persons listed on
Schedule A attached hereto (individually a "Stockholder" and collectively the
"Stockholders"). Capitalized terms not otherwise defined have the meanings
ascribed to them in Section 1.1.

     WHEREAS, the Company and the Stockholders have entered into this Agreement
as a condition to the Public Offering;

     WHEREAS, the Company has been an "S corporation" (as defined in Section
1361(a)(1) of the Code) for federal tax purposes since July 1, 1987;

     WHEREAS, the Company and the Stockholders understand that the Company's S
corporation status will terminate upon the date of the Public Offering (the
"Termination Date"), and, as a result, the Company will be a "C corporation" (as
defined in Section 1361(a)(2) of the Code) beginning on the Termination Date;

     WHEREAS, the Company will declare the AAA Dividend which will be payable as
soon as possible on or after the Closing Date;

     WHEREAS, the Company and the Stockholders wish to terminate this Agreement
such that it has no effect should the Public Offering not occur;

     NOW, THEREFORE, the parties agree as follows:


                                    ARTICLE I

                                   DEFINITIONS
                                   -----------

     1.1 DEFINITIONS. The following terms, as used herein, have the following
meanings:

         "AA Account" means the Company's "accumulated adjustments account," as
         defined in Section 1368(e)(1) of the Code, as of the close of business
         on the day before the Termination Date.

         "AAA Dividend" means the dividend to be declared by the Company of all
         of the undistributed AA Account.


<PAGE>   2



          "AAA Settlement Date" means the last day of the "post-termination
transition period," as defined in Section 1377(b) of the Code, of the Company,
except shall not include any extension of the post-termination transition period
pursuant to Section 1377(b)(l)(C) as the result of a determination that the
Company's election under Section 1362(a) had terminated earlier than the Closing
Date.

          "Closing Date" means the date on which the Public Offering closes.

          "Code" means the Internal Revenue Code of 1986, as amended.

          "C Short Year" means that portion of the S Termination Year of the
Company beginning on the Termination Date and ending on the last day of the S
Termination Year.

          "C Taxable Year" means any taxable year (or portion thereof) of the
Company, including the C Short Year, during which it is subject to taxation as a
C corporation as defined in Section 1361(a)(2) of the Code.

          "Final Determination" means the first to occur of

     (i)  the expiration of 30 days after IRS acceptance of a Waiver of
     Restrictions on Assessment and Collection of Deficiency of Tax and
     Acceptance of Overassessment on IRS Form 870 or 870-AD (or any successor
     comparable form or the expiration of a comparable period with respect to
     any comparable agreement or form under the laws of other jurisdictions);

     (ii) a decision, judgment, decree, or other order by a court of competent
     jurisdiction that is not subject to further judicial review and has become
     final;

     (iii)the execution of a closing agreement under Section 7121 of the Code
     or the acceptance by the IRS of an offer in compromise under Section 7122
     of the Code, or comparable agreements under the laws of other
     jurisdictions;

     (iv) the expiration of the time for filing a claim for refund or for
     instituting suit in respect for a claim for refund disallowed in whole or
     in part by the IRS or other relevant tax authority;

     (v)  any other final disposition of the tax liability for such period by
     reason of the expiration of the applicable statute of limitations; or

     (vi) any other event that the parties agree is final and irrevocable
     determination of the liability at issue.


                                        2

<PAGE>   3



          "Public Offering" means the public offering of the Company's Common
Stock pursuant to the Registration Statement on Form S-1 originally filed by the
Company with the Securities and Exchange Commission on January 28, 1999.

          "Record Date" means the day prior to the date upon which the Company's
Registration Statement on Form S-1, as amended, which was initially filed with
the Securities and Exchange Commission on January 28, 1999, is declared
effective by the Securities and Exchange Commission.

          "S Short Year" means that portion of the S Termination Year beginning
on the first day of such taxable year and ending on the day immediately
preceding the Termination Date.

          "S Taxable Year" means any taxable year (or portion thereof) of the
Company, including the S Short Year, during which it is subject to taxation as
an S corporation as defined in Section 1361(a)(1) of the Code.

          "S Termination Year" shall mean the fiscal year of the Company that
includes the Termination Date.

          "Taxing Authority" means the United States Internal Revenue Service
and any comparable state or foreign taxing authority.

          "Termination Date" means the date on which the S corporation status of
the Company will terminate pursuant to Section 1362(d) of the Code, which is
expected to be the Closing Date.


                                   ARTICLE II

           TERMINATION OF S CORPORATION STATUS, ALLOCATION OF INCOME,
           ----------------------------------------------------------
           DECLARATION OF AAA DIVIDEND AND ADJUSTMENT OF AAA DIVIDEND
           ----------------------------------------------------------

     2.1  TERMINATION OF S CORPORATION STATUS. The Company and the Stockholders
understand that the Company's S corporation status will terminate upon the
consummation of the Public Offering.

     2.2  PRO RATA ALLOCATION OF TAX ITEMS. The Company shall be required to
allocate the tax items described in Section 1362(e)(2)(A) of the Code between
the S Short Year and the C Short Year pursuant to the pro rata allocation rules
set forth in Section 1362(e)(2)(B) of the Code.

     2.3  DECLARATION OF AAA DIVIDEND. Prior to the Closing Date, the Company
shall declare the AAA Dividend, subject to the closing of the Public Offering,
to the

                                        3

<PAGE>   4



stockholders of record on the Record Date payable as soon as possible on or
after the Closing Date.

     2.4  ADJUSTMENT TO AAA DIVIDEND.

          (a) The parties acknowledge that the amount of the AAA Dividend will
be based on good faith determinations by the Company of the amount of AA Account
as of the Termination Date.

          (b) The parties agree that if the Company determines after the
Termination Date and on or before the AAA Settlement Date that the amount of the
AA Account as of the Termination Date does not equal the amount of the AAA
Dividend, then:

               (i)  if the amount of the AAA Dividend exceeds the amount of AA
          Account as of the Termination Date, the Stockholders who received the
          AAA Dividend shall thereafter remit to the Company their pro-rata
          share of such excess no later than thirty (30) days after receiving
          notice from the Company that such amount is payable; and

               (ii) if the amount of the AA Account as of Termination Date
          exceeds the amount of the AAA Dividend, the Company shall thereafter
          distribute to the Stockholders their pro-rata shares of such excess
          within thirty (30) days following the date the Company determines an
          amount is payable pursuant to this Section 2.4(b)(ii).

          (c) Any payment due under this section shall be increased by interest
on the amount of such payment computed from the date of the payment of the AAA
Dividend until the date of payment pursuant to this section. The interest rate
shall be the Prime Rate of BankBoston (or its successors) as adjusted from time
to time.


                                   ARTICLE III

                  TAX PAYMENTS AND INDEMNIFICATION OBLIGATIONS
                  -=------------------------------------------

     3.1  LIABILITY FOR TAXES INCURRED DURING S SHORT YEAR. Each Stockholder
covenants and agrees that: (i) the Stockholder will duly include, in his own
federal and state income tax returns, all items of income, gain, loss,
deduction, or credit attributable to the S Short Year in a manner consistent
with the Form 1120S and the schedules thereto (and the corresponding state
income tax forms and schedules) to be filed by the Company with respect to such
period; (ii) such returns shall be filed no later than the due date (including
extensions, if any) for filing such returns; and (iii)

                                        4

<PAGE>   5



each Stockholder shall pay any and all taxes required to be paid for its taxable
year that includes the S Short Year.

     3.2  LIABILITY FOR TAXES INCURRED DURING S SHORT YEAR AND C SHORT YEAR. The
Company covenants and agrees that: (i) the Company shall be responsible for and
shall effect the filing of all federal and state income tax returns for the
Company with respect to the S Short year and the C Short Year; (ii) such Company
returns shall be accurately prepared and timely filed; and (iii) the Company
shall pay any and all taxes required to be paid by the Company for the periods
covered by such returns as required by applicable law.

     3.3  STOCKHOLDERS INDEMNIFICATION OF COMPANY FOR TAX LIABILITIES.

          (a)  ADJUSTMENTS ATTRIBUTABLE TO COMPANY'S S STATUS. If, based on a
Final Determination, the Company is deemed to have been a C corporation for
federal, state or local income tax purposes during any period in which it
reported (or intends to report) its taxable income as an S corporation, each
Stockholder agrees to contribute to the capital of the Company, subject to the
limitations contained in the last sentence of this Section 3.3(a) and in Section
3.3(b), an amount necessary to hold the Company harmless from any taxes (net of
any refunds), penalties and interest arising from such Final Determination. Each
Stockholder's obligation under this Section 3.3(a) shall be several and not
joint and shall be limited to that percentage of the tax (net of any refunds),
penalties and interest due and payable by the Company equal to the fraction,
expressed as a percentage, the numerator of which is the total distributions to
such Stockholder made by the Company from July 1, 1987 through and including the
Termination Date, plus the AAA Dividend and any adjustment thereto pursuant to
this Agreement, and the denominator of which is the total distributions made by
the Company to all Stockholders from July 1, 1987 through and including the
Termination Date, plus the AAA Dividend and any adjustment thereto pursuant to
this Agreement.

          (b)  LIMIT ON INDEMNIFICATION AMOUNT. Any payment by a Stockholder to
the Company pursuant to this Section 3.3 shall not exceed the amount of the
total distributions made to such Stockholder by the Company from July 1, 1987
through and including the Termination Date, plus the AAA Dividend and any
adjustment thereto pursuant to this Agreement, reduced by any taxes paid or
payable by the Stockholders on the distributions and increased by any refund of
taxes and interest received by the Stockholders with respect to the Company's S
corporation earnings.

          (c)  TIME OF INDEMNIFICATION PAYMENT. The Stockholders shall
contribute to the capital of the Company amounts set forth in this Section 3.3
within thirty (30) days after notice from the Company that a payment is due by
the Company to the appropriate Taxing Authority.

                                        5

<PAGE>   6




     3.4  CONTESTS/COOPERATION.

          (a)  CONTESTS. Each of the Company and the Stockholders agree that (i)
in the event that any of them receives notice, whether orally or in writing, of
any federal, state, local or foreign tax examinations, claims, settlements,
proposed adjustments or related matters that may affect in any way the liability
of any of such persons whether under this Agreement or otherwise, it shall
within ten days notify the other parties in writing thereof (provided that any
failure to give such notice shall not reduce a party's right to indemnification
under this Agreement except to the extent of actual damage incurred by the other
parties as a result of such failure), and (ii) the Company shall be entitled at
its reasonable discretion and sole expense to handle, control and compromise or
settle the defense of any matter of the Company or any Stockholder which may
give rise to a liability under this Agreement or a limitation of liability under
this Agreement, provided that the Stockholders may participate in all
conferences, meetings or proceedings with respect to the issue.

          (b)  COOPERATION. The parties will make available to one another, as
reasonably requested, and to any Taxing Authority, all information, records or
documents relating to the liability for taxes covered by this Agreement and will
preserve such information, records or documents until the expiration of any
applicable statute of limitations or extensions thereof. The party requesting
such information shall reimburse the other party for all reasonable
out-of-pocket costs incurred in producing such information.

          (c)  CLAIMS FOR REFUND. Each party hereto agrees to file a properly
completed claim with the appropriate Taxing Authority for a refund or an
abatement of taxes paid with respect to any matter which may give rise to a
liability under Section 3.4 of this Agreement.

     3.5  COSTS. Except to the extent otherwise provided herein, each party
shall bear its own costs in administering this Agreement.

     3.6  CORRECTION OF A FINAL DETERMINATION. In the event a party makes a
payment pursuant to this Agreement based on the expected outcome of a Final
Determination which subsequently is determined to have been incorrect, the
parties shall adjust the payments hereunder in order to reflect the subsequent
determination as if it was the Final Determination upon which the original
payment was based.


                                        6

<PAGE>   7




                                   ARTICLE IV

                                  MISCELLANEOUS
                                  -------------


     4.1  DISPUTES. If the parties are, after negotiation in good faith, unable
to agree upon the appropriate application of this Agreement, the controversy
shall be settled by the accounting firm remaining on the list of firms set forth
on Schedule B hereto after the Company and the representative of the
Stockholders, commencing with the Company, shall have objected seriatim to the
other firms of the list (the "Accounting Firm"). The decision of the Accounting
Firm shall be final, and each of the Company and the Stockholders agree
immediately to pay to the other any amount due under this Agreement pursuant to
such decision. The expenses of the Accounting firm shall be borne one-half by
the Company and one-half by the Stockholders, in the same proportion that the
Stockholders share liability under Section 3.3 hereof, unless the Accounting
Firm specifies otherwise.

     4.2  COUNTERPARTS. This Agreement may be executed in several counterparts,
each of which shall be deemed an original, but all of which counterparts
collectively shall constitute an instrument representing the Agreement between
the parties hereto.

     4.3  CONSTRUCTION OF TERMS. Nothing herein expressed or implied is
intended, or shall be construed, to confer upon or give any person, firm or
corporation, other than the parties hereto or their respective successors and
assigns, any rights or remedies under or by reason of this Agreement.

     4.4  GOVERNING LAW. This Agreement and the legal relations between the
parties hereto shall be governed by and construed in accordance with the
substantive laws of the Commonwealth of Massachusetts without regard to
Massachusetts choice of law rules.

     4.5  AMENDMENT AND MODIFICATION. This Agreement may be amended, modified or
supplemented only by a written agreement executed by the parties.

     4.6  ASSIGNMENT. This Agreement and all of the provisions hereof shall be
binding upon and inure to the benefit of the parties hereto and their respective
successors and permitted assigns, but neither this Agreement nor any of the
rights, interests or obligations hereunder shall be assigned by any of the
parties hereto without the prior written consent of the other parties, nor is
this Agreement intended to confer upon any other person except the parties any
rights or remedies hereunder.

     4.7  INTERPRETATION. The title, article and section headings contained in
this Agreement are solely for the purpose of reference, are not part of the
agreement of

                                        7

<PAGE>   8



the parties and shall not in any way affect the meaning or interpretation of
this Agreement.

     4.8  SEVERABILITY. In the event that any one or more of the provisions of
this Agreement shall be held to be illegal, invalid or unenforceable in any
respect, the same shall not in any respect affect the validity, legality or
enforceability of the remainder of this Agreement, and the parties shall use
their best efforts to replace such illegal, invalid or unenforceable provisions
with an enforceable provision approximating, to the extent possible, the
original intent of the parties.

     4.9  ENTIRE AGREEMENT. This Agreement embodies the entire agreement and
understanding of the parties hereto in respect to the subject matter contained
herein. There are no representations, promises, warranties, covenants, or
undertakings, other than those expressly set forth or referred to herein. This
Agreement supersedes all prior agreements and the understandings between the
parties with respect to such subject matter.

     4.10 INTEREST ON OVERDUE PAYMENTS. Any payment pursuant to this Agreement
not made when due under this Agreement shall bear interest at the rate of the
Prime Rate of BankBoston (or its successors), as published from time to time.

     4.11 SETOFF. All payments to be made by any party under this Agreement
shall be made without setoff, counterclaim or withholding, all of which are
expressly waived.

     4.12 NOTICES. All notices provided for in this Agreement shall be validly
given if in writing and delivered personally or sent by registered mail, postage
prepaid

     if to the Company, to:

     Chief Financial Officer
     MKS Instruments, Inc.
     6 Shattuck Road
     Andover, MA  01810

     copy to:
     Richard S. Chute
     Hill & Barlow
     One International Place
     Boston, MA  02110


                                        8

<PAGE>   9



     if to any Stockholder, to:

     Name of Stockholder
     c/o John R. Bertucci
     President
     MKS Instruments, Inc.
     6 Shattuck Road
     Andover, MA  01810

     4.13 TERMINATION OF AGREEMENT. This Agreement shall terminate and be void,
as if it never had been executed, if the Closing Date shall occur after April
30, 1999.


     IN WITNESS WHEREOF, the parties have executed this Agreement as of the date
first written above.

                                        MKS INSTRUMENTS, INC.



                                        By:
                                             -----------------------------------
                                             Chief Financial Officer

                                        STOCKHOLDERS


                                        ----------------------------------------
                                        Michael F. Brunelli

                                        Claire R. Bertucci JCB Retained
                                        Annuity Trust 1998

                                        By:
                                             -----------------------------------
                                             Trustee

                                             Claire R. Bertucci CBS Retained
                                             Annuity Trust 1998

                                        By:
                                             -----------------------------------
                                             Trustee


                                        9

<PAGE>   10




                                        John R. Bertucci JCB Retained Annuity
                                        Trust 1998

                                        By:
                                             -----------------------------------
                                             Trustee
                                        John R. Bertucci CBS Retained
                                        Annuity Trust 1998

                                        By:
                                             -----------------------------------
                                             Trustee

                                        John R. Bertucci Family Retained
                                        Annuity Trust 1998

                                        By:
                                             -----------------------------------
                                             Trustee

                                        ----------------------------------------
                                        John J. Sullivan

                                        ----------------------------------------
                                        Cheryl A. Sweeting

                                        ----------------------------------------
                                        Thomas J. Sullivan

                                        ----------------------------------------
                                        John F. Sullivan

                                        ----------------------------------------
                                        Kathleen M. Davis

                                        John J. Sullivan Retained Annuity
                                        Trust of 1997

                                        By:
                                             -----------------------------------
                                             Trustee

                                        ----------------------------------------
                                        Claire R. Bertucci


                                       10

<PAGE>   11




                                        Claire R. Bertucci JCB Retained
                                        Annuity Irrevocable Trust of 1997

                                        By:
                                             -----------------------------------
                                             Trustee

                                        Claire R. Bertucci CBS Retained
                                        Annuity Irrevocable Trust of 1997

                                        By:
                                             -----------------------------------
                                             Trustee


                                        ----------------------------------------
                                        John R. Bertucci

                                        John R. Bertucci JCB Retained Annuity
                                        Irrevocable Trust of 1997

                                        By:
                                             -----------------------------------
                                             Trustee

                                        John R. Bertucci CBS Retained
                                        Annuity Irrevocable Trust of 1997

                                        By:
                                             -----------------------------------
                                             Trustee

                                        John R. Bertucci Family Retained
                                        Annuity Irrevocable Trust of 1997

                                        By:
                                             -----------------------------------
                                             Trustee

                                        Claire R. Bertucci 2nd Family Trust of
                                        December 15, 1986 f/b/o Janet C.
                                        Bertucci

                                        By:
                                             -----------------------------------
                                             Trustee


                                       11

<PAGE>   12




                                        Claire R. Bertucci 2nd Family Trust of
                                        December 15, 1986 f/b/o Carol B.
                                        Bertucci

                                        By:  -----------------------------------
                                             Trustee

                                        John R. Bertucci 2nd Family Trust of
                                        December 15, 1986 f/b/o Janet C.
                                        Bertucci

                                        By:  -----------------------------------
                                             Trustee

                                        John R. Bertucci 2nd Family Trust of
                                        December 15, 1986 f/b/o Carol B.
                                        Bertucci

                                        By:  -----------------------------------
                                             Trustee


                                        ----------------------------------------
                                        John S. Nelson





                                       12

<PAGE>   13



                                   SCHEDULE A

                              LIST OF STOCKHOLDERS


Michael F. Brunelli

Claire R. Bertucci JCB Retained Annuity Trust 1998

Claire R. Bertucci CBS Retained Annuity Trust 1998

John R. Bertucci JCB Retained Annuity Trust 1998

John R. Bertucci CBS Retained Annuity Trust 1998

John R. Bertucci Family Retained Annuity Trust 1998

John J. Sullivan

Cheryl A. Sweeting

Thomas J. Sullivan

John F. Sullivan

Kathleen M. Davis

John J. Sullivan Retained Annuity Trust of 1997

Claire R. Bertucci

Claire R. Bertucci JCB Retained Annuity Irrevocable Trust of 1997

Claire R. Bertucci CBS Retained Annuity Irrevocable Trust of 1997

John R. Bertucci

John R. Bertucci JCB Retained Annuity Irrevocable Trust of 1997

John R. Bertucci CBS Retained Annuity Irrevocable Trust of 1997

John R. Bertucci Family Retained Annuity Irrevocable Trust of 1997

Claire R. Bertucci 2nd Family Trust of December 15, 1986 f/b/o Janet C. Bertucci

                                       13

<PAGE>   14



Claire R. Bertucci 2nd Family Trust of December 15, 1986 f/b/o Carol B. Bertucci

John R. Bertucci 2nd Family Trust of December 15, 1986 f/b/o Janet C. Bertucci

John R. Bertucci 2nd Family Trust of December 15, 1986 f/b/o Carol B. Bertucci

John S. Nelson

                                       14

<PAGE>   15



                                   SCHEDULE B

Arthur Anderson LLP

KPMG Peat Marwick LLP

Deloitte & Touche LLP

Ernst & Young LLP

PricewaterhouseCoopers LLP





                                       15




<PAGE>   1
                                                                   EXHIBIT 10.26

                             CONTRIBUTION AGREEMENT


         This CONTRIBUTION AGREEMENT (the "Agreement") is entered into as of
March 18, 1999 between MKS INSTRUMENTS, INC., a Massachusetts corporation (the
"Parent"), and the persons listed on SCHEDULE A attached hereto (individually a
"Stockholder" and collectively the "Stockholders"). Each of the Stockholders
owns shares of capital stock of the Parent and MKS International, Inc., a
Massachusetts corporation and a subsidiary of Parent (the "Subsidiary").

         WHEREAS, the Parent has filed a Registration Statement on Form S-1 with
the Securities and Exchange Commission in connection with a firm commitment
underwritten public offering (the "IPO") of shares of its common stock, no par
value ("Common Stock");

         WHEREAS, in connection with the IPO, the Parent will list its Common
Stock for quotation on the Nasdaq National Market System (the "Listing");

         WHEREAS, the Listing is intended to facilitate a public trading market 
in the Common Stock of the Company;

         WHEREAS, the Stockholders acknowledge that such a public trading market
would add to the value of the shares of Common Stock owned by them; and

         WHEREAS, the Stockholders and the Parent believe that, in order to
facilitate the IPO, the Subsidiary, should be wholly owned by the Parent;

         NOW, THEREFORE, for good and valuable consideration, the sufficiency of
which is hereby acknowledged, the undersigned hereby agree as follows:

         1. CONTRIBUTION OF SHARES OF SUBSIDIARIES. Subject to the terms and
conditions of this Contribution Agreement, each of the Stockholders hereby
agrees that, effective immediately prior to the closing of the IPO (the
"Closing"), all of his, her or its interest in all shares of the Subsidiary
owned by him, her or it (the "Shares") shall be contributed, transferred,
conveyed, assigned and delivered to the Parent or its designee. At the Closing
each Stockholder shall deliver to the Parent certificates evidencing the Shares
owned by such Stockholder duly endorsed in blank or with stock powers duly
executed by such Stockholder.

         2. FURTHER ASSURANCES. At any time and from time to time after the
Closing, at the Parent's request and without consideration, each of the
Stockholders shall promptly execute and deliver such instruments of
contribution, transfer, conveyance, assignment and confirmation, and take all
such other action as the Parent 

<PAGE>   2


may reasonably request, more effectively to transfer, convey and assign to the
Parent or its designee, and to confirm the Parent's or its designee's title to,
all of the Shares owned by such Stockholder, and to assist the Parent in
exercising all rights with respect thereto and to carry out the purpose and
intent of this Agreement.

         3. REPRESENTATIONS OF THE STOCKHOLDERS. Each Stockholder severally
represents and warrants to the Parent as follows:

         (a) Except as set forth on SCHEDULE B attached hereto, such Stockholder
has good and marketable title to the Shares which are to be transferred to the
Parent by such Stockholder pursuant hereto, free and clear of any and all
covenants, conditions, restrictions, voting trust arrangements, liens, charges,
encumbrances, options and adverse claims or rights whatsoever. SCHEDULE A
attached hereto sets forth a true and correct description of all Shares owned by
such Stockholder.

         (b) Except as set forth on SCHEDULE B attached hereto, such Stockholder
has the full right, power and authority to enter into this Agreement and to
transfer, convey and assign to the Parent or its designee at the Closing the
Shares to be contributed by such Stockholder hereunder and, upon consummation of
the IPO, the Parent or its designee will acquire from such Stockholder good and
marketable title to such Shares, free and clear of all covenants, conditions,
restrictions, voting trust arrangements, liens, charges, encumbrances, options
and adverse claims or rights whatsoever.

         (c) Except as set forth on SCHEDULE B attached hereto, such Stockholder
is not a party to, subject to or bound by any agreement or any judgment, order,
writ, prohibition, injunction or decree of any court or other governmental body
which would prevent the execution or delivery of this Agreement by such
Stockholder or the transfer, conveyance and assignment of the Shares to be
contributed by such Stockholder to the Parent or its designee pursuant to the
terms hereof.

         4.       MISCELLANEOUS.

                  4.1 COUNTERPARTS. This Agreement may be executed in several
counterparts, each of which shall be deemed an original, but all of which
counterparts collectively shall constitute an instrument representing the
Agreement between the parties hereto.

                  4.2 CONSTRUCTION OF TERMS. Nothing herein expressed or implied
is intended, or shall be construed, to confer upon or give any person, firm or
corporation, other than the parties hereto or their respective successors and
assigns, any rights or remedies under or by reason of this Agreement.

                                      -2-
<PAGE>   3


                  4.3 GOVERNING LAW. This Agreement and the legal relations
between the parties hereto shall be governed by and construed in accordance with
the substantive laws of the Commonwealth of Massachusetts without regard to
Massachusetts choice of law rules.

                  4.4 AMENDMENT AND MODIFICATION. This Agreement may be amended,
modified or supplemented only by a written agreement executed by the parties.

                  4.5 ASSIGNMENT. This Agreement and all of the provisions
hereof shall be binding upon and inure to the benefit of the parties hereto and
their respective successors and permitted assigns, but neither this Agreement
nor any of the rights, interests or obligations hereunder shall be assigned by
any of the Stockholders without the prior written consent of the Parent.

                  4.6 INTERPRETATION. The section headings contained in this
Agreement are solely for the purpose of reference, are not part of the agreement
of the parties and shall not in any way affect the meaning or interpretation of
this Agreement.

                  4.7 SEVERABILITY. In the event that any one or more of the
provisions of this Agreement shall be held to be illegal, invalid or
unenforceable in any respect, the same shall not in any respect affect the
validity, legality or enforceability of the remainder of this Agreement, and the
parties shall use their best efforts to replace such illegal, invalid or
unenforceable provisions with an enforceable provision approximating, to the
extent possible, the original intent of the parties.

                  4.8 ENTIRE AGREEMENT. This Agreement embodies the entire
agreement and understanding of the parties hereto in respect to the subject
matter contained herein. There are no representations, promises, warranties,
covenants, or undertakings, other than those expressly set forth or referred to
herein. This Agreement supersedes all prior agreements and the understandings
between the parties with respect to such subject matter.

                  4.9 NOTICES. All notices provided for in this Agreement shall
be validly given if in writing and delivered personally or sent by registered
mail, postage prepaid

                                      -3-
<PAGE>   4

         if to the Parent, to:

         President
         MKS Instruments, Inc.
         6 Shattuck Road
         Andover, MA  01810

         copy to:
         Richard S. Chute
         Hill & Barlow
         One International Place
         Boston, MA  02110

         If to any Stockholder, to the address appearing under such
Stockholder's name on SCHEDULE A attached hereto, or at such other address
provided by such Stockholder in writing to the Parent.

                  4.10 TERMINATION OF AGREEMENT. This Agreement shall terminate
and be void, as if it never had been executed, if the Closing shall not have
occurred on or before July 31, 1999.

         IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first written above.


                                       MKS INSTRUMENTS, INC.:


                                       By: 
                                           -------------------------------------
                                            Ronald C. Weigner, Vice President 
                                            and Chief Financial Officer


                                       STOCKHOLDERS:


                                       -----------------------------------------
                                       John R. Bertucci


                                       -----------------------------------------
                                       Claire R. Bertucci


                                      -4-
<PAGE>   5

                                       -----------------------------------------
                                       John J. Sullivan


                                       Claire R. Bertucci Second Family Trust of
                                       December 15, 1986 FBO Carol B. Bertucci


                                       By:
                                           -------------------------------------
                                           John R. Bertucci, Trustee


                                       By:
                                           -------------------------------------
                                           Thomas H. Belknap, Trustee


                                       Claire R. Bertucci Second Family Trust of
                                       December 15, 1986 FBO Janet C. Bertucci


                                       By:
                                           -------------------------------------
                                           John R. Bertucci, Trustee


                                       By:
                                           -------------------------------------
                                           Thomas H. Belknap, Trustee


                                       John R. Bertucci Second Family Trust of 
                                       December 15, 1986 FBO Carol B. Bertucci


                                       By:
                                           -------------------------------------
                                           Claire R. Bertucci, Trustee

                                       By:
                                           -------------------------------------
                                           Richard S. Chute, Trustee

                                       John R. Bertucci Second Family Trust of 
                                       December 15, 1986 FEC Janet C. Bertucci


                                      -5-
<PAGE>   6


                                       By:  
                                           -------------------------------------
                                           Claire R. Bertucci, Trustee


                                       By:
                                           -------------------------------------
                                           Richard S. Chute, Trustee


                                      -6-

<PAGE>   7

                                   SCHEDULE A

                                  STOCKHOLDERS
<TABLE>
<CAPTION>
                                                               MKS International
                                                                   Shares Owned
<S>                                                             <C>

John R. Bertucci                                                     2,315.0
  c/o MKS Instruments, Inc.
  6 Shattuck Road
  Andover, MA 01810

Claire R. Bertucci                                                   2,205.0
  c/o MKS Instruments, Inc.
  6 Shattuck Road
  Andover, MA 01810

Claire R. Bertucci Second Family Trust of                              233.5
December 15, 1986 FBO Carol B. Bertucci
  c/o Hill & Barlow
  One International Place
  Boston, MA 02110

Claire R. Bertucci Second Family Trust of                              233.5
December 15, 1986 FBO Janet C. Bertucci
  c/o Hill & Barlow
  One International Place
  Boston, MA 02110

John R. Bertucci Second Family Trust of                                233.5
December 15, 1986 FBO Carol B. Bertucci
  c/o Hill & Barlow
  One International Place
  Boston, MA 02110

John R. Bertucci Second Family Trust of                                233.5
December 15, 1986 FBO Janet C. Bertucci
  c/o Hill & Barlow
  One International Place
  Boston, MA 02110

John J. Sullivan                                                       250.0
  c/o MKS Instruments, Inc.
  6 Shattuck Road
  Andover, MA 01810
</TABLE>




<PAGE>   8


                                   SCHEDULE B





<PAGE>   1

   

                       CONSENT OF INDEPENDENT ACCOUNTANTS


     We consent to the inclusion in this Amendment No. 3 to the registration
statement on Form S-1 (File No. 333-71363) of our report dated January 22, 1999,
except for the information in the first and second paragraph of Note 13, as to
which the date is January 28, 1999 and February 24, 1999, respectively, on our
audit of the consolidated financial statements and our report dated January 22,
1999, on our audit of the financial statement schedule of MKS Instruments, Inc.
We consent to the references to our firm under the captions "Experts" and
"Selected Consolidated Financial Data."



                                                     PricewaterhouseCoopers LLP


Boston, Massachusetts
March 22, 1999


    


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