ATMI INC
S-3, 2000-01-14
INDUSTRIAL INORGANIC CHEMICALS
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<PAGE>

    As filed with the Securities and Exchange Commission on January 13, 2000

                                             Registration No. 333-
================================================================================

                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549

                              ___________________

                                   FORM S-3
                            REGISTRATION STATEMENT
                                     UNDER
                          THE SECURITIES ACT OF 1933

                              ___________________

                                  ATMI, INC.
            (Exact name of registrant as specified in its charter)

              Delaware                             06-1481060
      (State or other jurisdiction of            (I.R.S. Employer
      incorporation or organization)             Identification No.)

                              ___________________

                               7 Commerce Drive
                          Danbury, Connecticut 06810
                                (203) 794-1100
  (Address, including zip code, and telephone number, including area code, of
                   registrant's principal executive offices)

                              ___________________

                           Eugene G. Banucci, Ph.D.
                            Chief Executive Officer
                                  ATMI, Inc.
                               7 Commerce Drive
                          Danbury, Connecticut 06810
                                (203) 794-1100

(Name, address, including zip code, and telephone number, including area code,
                             of agent for service)

                              __________________

                                   Copy to:
                             Donna L. Brooks, Esq.
                             Shipman & Goodwin LLP
                               One American Row
                         Hartford, Connecticut 06103
                                (860) 251-5000

                              __________________

  Approximate date of commencement of proposed sale to the public:  From time to
time after the effective date of this Registration Statement.

  If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box. [_]

  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, other than securities offered only in connection with dividend or interest
reinvestment plans, check the following box. [X]

  If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please 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 delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. [ ]

                              __________________

                        CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
=====================================================================================================================
                                                          Proposed                Proposed
    Title of Each Class             Amount                 Maximum                 Maximum
    of Securities to be              to be              Offering Price            Aggregate             Amount of
        Registered                Registered            per Share (1)           Offering Price       Registration Fee
- ---------------------------------------------------------------------------------------------------------------------
<S>                               <C>                   <C>                     <C>                  <C>
Common Stock, par value
$.01 ..................           1,543,282             $30.78                   $47,502,219.96      $12,540.59
=====================================================================================================================
</TABLE>

(1) Estimated solely for the purpose of calculating the registration fee
    pursuant to Rule 457(c) under the Securities Act of 1933 based on the
    average of the high and low prices on the Nasdaq National Market on January
    6, 2000.

                              ___________________

    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 this Registration Statement shall become
effective on such date as the Commission, acting pursuant to said Section 8(a),
may determine.

================================================================================
<PAGE>

                 Subject to Completion, Dated January 13, 2000

PROSPECTUS
- ----------
                               1,543,282 Shares

                                  ATMI, INC.

                                 Common Stock

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

  The stockholders of ATMI listed on page 10 of this prospectus are offering and
selling a total of 1,543,282 shares of ATMI common stock under this prospectus.
The selling stockholders will receive all of the net proceeds from the sale of
the shares.  The selling stockholders will pay all underwriting discounts and
selling commissions, if any, applicable to the sale of the shares.  We will not
receive any proceeds from the sale of the shares.

  Our common stock is traded on the Nasdaq National Market under the symbol
"ATMI."  On January 12, 2000, the closing sale price of our common stock was
$31.5625 per share.

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

  Investing in our common stock involves a high degree of risk.  See "Risk
Factors" beginning on page 5.

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

  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 representations to the contrary is a
criminal offense.

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

    The date of this Prospectus is                                         .
<PAGE>

                               TABLE OF CONTENTS

<TABLE>
<CAPTION>

                                                                            Page
                                                                            ----
<S>                                                                         <C>

Where You Can Find More Information........................................  3
The Company................................................................  4
Special Note Regarding Forward-Looking Information.........................  5
Risk Factors...............................................................  5
Use of Proceeds............................................................ 10
Selling Stockholders....................................................... 10
Plan of Distribution....................................................... 12
Legal Matters.............................................................. 12
Experts.................................................................... 12
</TABLE>

  You should rely only on the information incorporated by reference or contained
in this prospectus.  We have not authorized anyone to provide you with different
information from that contained in this prospectus.  The selling stockholders
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.

                      WHERE YOU CAN FIND MORE INFORMATION

  We file annual, quarterly and current reports, proxy statements and other
documents with the Securities and Exchange Commission.  You may read and copy
any document we file at the Commission's public reference rooms at Judiciary
Plaza, 450 Fifth Street, N.W., Washington, D.C. 20549, Seven World Trade Center,
Suite 1300, New York, New York 10048 and Citicorp Center, 500 West Madison
Street, Suite 1400, Chicago, Illinois 60661.  Please call the Commission at 1-
800-SEC-0330 for further information on public reference rooms.  Our reports,
proxy statements and other information filed with the Commission are also
available to the public from our web site at http://www.atmi.com or at the
Commission's web site at http://www.sec.gov.

  This prospectus is part of a registration statement that we filed with the
Commission.  The registration statement contains more information than this
prospectus regarding ATMI and its common stock, including certain exhibits and
schedules.  You can get a copy of the registration statement from the sources
listed above.

  The Commission allows us to "incorporate by reference" the information we file
with them, which means that we can disclose important information to you by
referring you to those documents.  The information incorporated by reference is
considered to be part of this prospectus, and later information that we file
with the Commission will automatically update and supersede this information.
We incorporate by reference the documents listed below and any future filings
made by us with the Commission under Section 13(a), 13(c), 14 or 15(d) of the
Securities Exchange Act of 1934 until the selling stockholders sell all of the
shares:

          (1)  Annual Report on Form 10-K for the year ended December 31, 1998
               (File No. 0-30130);

          (2)  Quarterly Reports on Form 10-Q for the quarters ended March 31,
               1999, June 30, 1999 and September 30, 1999;

          (3)  Current Report on Form 8-K/A dated May 31, 1999 and Current
               Report on Form 8-K/A dated November 29, 1999; and

          (4)  The description of the common stock of ATMI's predecessor
               registrant, Advanced Technology Materials, Inc., contained in its
               registration statement on Form 8-A filed on October 23, 1993,
               which description was amended on Form 8-A/A filed on November 11,
               1993, and any amendment or report filed to update this
               description.


                                      -3-
<PAGE>

  You may request a copy of these documents, at no cost, by writing or
telephoning us at the following address:

                                  ATMI, Inc.
                               7 Commerce Drive
                          Danbury, Connecticut  06810
                             Attn:  Dean Hamilton
                                (203) 794-1100

                                  THE COMPANY

  We are a leading supplier of materials and equipment used in the manufacture
of semiconductor devices. Our specific focus is proprietary and patented
products that are used in the manufacture of the thin films that are deposited
on semiconductor wafers to make them functional as complex devices. We currently
provide:


          .  a broad range of ultrahigh-purity semiconductor materials;

          .  semiconductor materials packaging and delivery systems that bring
             materials to the processing equipment;

          .  sensors for the workplace and environment that detect materials
             as they move through the workplace;

          .  point-of-use environmental equipment that abates materials; and

          .  specialty thin film deposition services that provide coated wafers
             directly to our customers.

  Over the last four years, we have achieved a leadership position in each of
our target markets by providing a more complete line of products than our
competitors.  Our strategy is to continue our growth through product line
expansion in each of our existing markets and to leverage our core technology to
create new high growth businesses.  Our customers include most of the leading
semiconductor manufacturers in the world.

  We have capitalized on the growth of the semiconductor industry in general by
providing leading edge products and services in each of our target markets.  We
have organized our operations along two business segments:  ATMI Materials and
ATMI Technologies.  ATMI Materials provides all of ATMI's products that are used
in the semiconductor manufacturing process and the packaging and delivery
systems that are used to get them there.  ATMI Technologies provides products
that sense and environmentally control these materials while also providing
manufacturing services to those end users that do not wish to perform certain
thin film manufacturing processes in-house.  ATMI Technologies is also
responsible for ATMI's venture and government contracting activities.

  ATMI's products are sold under a number of brand names as a result of its
strategy of rapid growth through a combination of internal development and the
acquisition of complementary product lines. Key products developed internally
are: the SDS gas delivery system, which stores dangerous gases as solids in
cylinders, providing increased safety and substantially greater operating
efficiencies; the VaporSource liquid delivery system and a number of related
next generation thin film materials; several products that now comprise the
Ecosys environmental equipment business; and the Emosyn line of smart card
integrated circuits based on a proprietary materials and design methodology.
Recently acquired product lines include: the NOWPak and Newform high performance
packaging and dispensing systems; the ACSI photolithography stripping and
chemical mechanical processing materials, the Delatech line of environmental
abatement equipment; and the MST and Telosense lines of materials sensing
devices.

  We believe this portfolio of products makes ATMI a leader in its marketplace.
We believe that we have the unique ability to deliver complete "materials
solutions" to our customers, i.e. specialty materials, systems that

                                      -4-
<PAGE>

deliver them to the processing equipment, devices that measure their presence in
the workplace and the environment and equipment that removes them from the
environment.

  We are a Delaware corporation organized in 1997 and are the successor
registrant to Advanced Technology Materials which was incorporated in
Connecticut in 1986 and reincorporated in Delaware in 1987.  Our principal
executive offices are located at 7 Commerce Drive, Danbury, Connecticut  06810,
and our telephone number is (203) 794-1100.

              SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION

  This prospectus contains or incorporates "forward-looking statements" within
the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the
Securities Exchange Act of 1934.  You can identify these statements by forward-
looking words such as  "believes," "anticipates," "plans," "expects," "may,"
"will," "intends," "estimates" and similar words.  We can give no assurance that
we will actually achieve these plans, intentions or expectations.  Our actual
results could differ materially from the plans, intentions and expectations
disclosed in these forward-looking statements.  We have included important
factors in the cautionary statements below that we believe could cause our
actual results to differ materially from our forward-looking statements.  We do
not intend to update information contained in any of our forward-looking
statements.


                                  RISK FACTORS

  You should carefully consider the following risks, together with other
information contained or incorporated by reference in this prospectus, before
making an investment decision.  Our business, operating results and financial
condition could be adversely affected by any of the following risks.  The
trading price of our common stock could decline due to any of these risks, and
you could lose all or part of your investment.

Our quarterly operating results may fluctuate

  Our quarterly operating results may fluctuate in the future as a result of a
number of factors, including:

  .  the general demand for semiconductors;

  .  the cyclical nature of the semiconductor manufacturing equipment market;

  .  our product mix;

  .  our success in developing, introducing and shipping new products;

  .  competition;

  .  the timing of significant orders from, and shipments to, customers;

  .  the timing and market acceptance of new products;

  .  the timing and amount of bonus incentive payments to employees; and

  .  the effect of taxes and various non-recurring expenses.

As a result, our operating results in any quarter are not necessarily a good
predictor of our results for any future period.  In the future, we will likely
experience quarterly or annual fluctuations.  In one or more future quarters,
our

                                      -5-
<PAGE>

operating results may fall below the expectations of public market analysts
or investors, and the price of our common stock could decline significantly.

Our business could be adversely affected if we are unable to integrate
businesses we acquire

  During the past three years, we have acquired other companies, including
Advanced Delivery and Chemical Systems, Lawrence Semiconductor Laboratories, NOW
Technologies, Delatech, Advanced Chemical Systems International, TeloSense,
Newform and MST Analytics.  If opportunities exist, we intend to acquire other
businesses that we believe are strategic.  We may not achieve the anticipated
benefits from any acquisition, including our acquisitions of ADCS, LSL, NOW,
Delatech, ACSI, TeloSense, Newform and MST Analytics, unless we successfully
combine the acquired businesses with those of ATMI in a timely and efficient
manner.  The integration of acquisitions requires substantial attention from
management.  The diversion of the attention of management, and any difficulties
encountered in the transition process, could negatively impact our business,
operating results and financial condition.  In addition, the process of
integrating various businesses could cause the interruption of, or a loss of
momentum in, the activities of some or all of these businesses as well as our
ongoing business.

We may have difficulty managing our growth

  We have grown and intend to continue to grow our business.  The management of
our growth requires qualified personnel, systems and other resources.  Our
future success will depend in part on our ability to attract and retain highly
skilled scientific, technical, managerial and marketing personnel.  Competition
for such personnel in the semiconductor industry is intense, and our competitors
are often larger and more established than we are.  We may not be successful in
attracting and retaining qualified personnel.  In addition, our expansion may
also significantly strain operational, management, financial, sales and
marketing and other resources.  To manage growth effectively, we must continue
to enhance our information technology infrastructure, systems and controls and
successfully expand, train and manage our employee base.  We may not be able to
manage this expansion effectively, including providing satisfactory levels of
customer service and technical support.

Our dependence on the semiconductor market makes us vulnerable to industry
downturns

  Substantially all of our sales are to customers in the worldwide semiconductor
industry.  As a result, our operating results are materially dependent upon
economic and business conditions in the semiconductor industry.  The
semiconductor industry and the semiconductor equipment industry in particular
have been highly cyclical and have experienced periods of overcapacity at
various times, resulting in significantly reduced demand for semiconductor
materials, capital equipment and wafer processing services.  Any industry
downturn will likely reduce our revenues and earnings.

We face risks from international operations and sales

  International sales have accounted for up to 31.9% of our annual revenues over
the last three years.  We anticipate that international sales will continue to
account for a significant portion of our revenues.  As a result, our operations
are subject to risks inherent in international business activities, including:

  .  export controls;

  .  unexpected changes in legal and regulatory requirements;

  .  policy changes affecting the markets for semiconductor technology;

  .  changes in tariffs, exchange rates and other barriers;

  .  political and economic instability;

                                      -6-
<PAGE>

  .  difficulties in accounts receivable collection;

  .  difficulties in managing resellers or representatives;

  .  difficulties in staffing and managing international operations;

  .  difficulties in protecting our intellectual property outside the United
     States;

  .  seasonality of sales; and

  .  potentially adverse tax consequences.

  Although our sales to date have been predominantly denominated in U.S.
dollars, the value of the U.S. dollar in relation to other currencies may also
adversely affect our sales to customers outside the United States.  In addition,
the recent acquisitions of Newform and MST have increased the amount of our
operations conducted in Europe with European currency.  Also, expenses and
revenues of a portion of certain of our businesses are denominated in South
Korean, German or Belgian currency and are exposed to risks customarily
associated with currency fluctuations.  Any significant volatility in South
Korean, German or Belgian currency, as it relates to invested capital in our
respective businesses in those countries, could have a material adverse impact
on stockholders' equity as reflected in currency translation adjustments in our
financial statements.  We do not hedge our exposure with respect to such
fluctuations.  To the extent that we expand our international operations or
change our pricing practices to denominate prices in other currencies, we will
be exposed to increased risks of currency fluctuations.

  Certain of our businesses conduct a material portion of their activities in
Asia, including South Korea, Taiwan and Japan.  Volatile economic conditions in
the region or of those countries, or instability in the currency of those
countries or in those countries' capital markets, may negatively impact our
revenues and earnings.

Our revenues would suffer if we lost several key customers

  Sales to our top five customers have accounted for up to 12% of our revenues
over the last three years.  Our relationships with these customers are subject
to various risks, including:

  .  termination, reduction or modification in the event of changes in the
     customer's requirements or budgetary constraints;

  .  risks of potential disclosure of our confidential information to third
     parties; and

  .  the failure or inability of a customer to perform its prime contract with
     its customer.

Our revenues would suffer if our major customers reduce their purchases of our
products or services or secure alternative sources for products or services.

We may have difficulty responding to changing and competing technologies

  Semiconductor technology, materials and processes change rapidly.  We may not
be successful if we cannot keep pace with such advances.  We are evaluating a
number of new opportunities to commercialize our core technology, including the
commercialization of smart card devices employing our advanced dielectric
material technology, but these opportunities may not lead to commercial products
that are timely or competitive.  The technological advances of others may render
our current products or development efforts obsolete, and other equipment or
materials may prove more advantageous to customers in the markets we serve.  In
addition, certain of our current technologies face competition from other
existing technologies which may be superior to or more cost-effective than our
current technologies and products.

                                      -7-
<PAGE>

We face significant competition from a variety of sources

  The markets for semiconductor thin film materials and delivery systems,
environmental equipment and epitaxial deposition services are intensely
competitive.  A number of domestic and international companies engage in
commercial activities in the markets we serve.  Many of these companies have
substantially greater financial, research and development, manufacturing and
marketing resources than we do.  In addition, as this industry evolves, other
competitors may emerge.  To remain competitive, we must continue to invest in
and focus upon research and development and product and process innovation.  We
may not be successful if we cannot compete on:

  .  price;

  .  technical capabilities;

  .  quality;

  .  customer service; and

  .  the ability to provide full market-basket solutions to customers that are
     increasingly seeking to streamline their vendor relationships.

Our business could be adversely affected if we cannot protect our proprietary
technology

  Our proprietary technology aids our ability to compete effectively with other
companies.  Although we have been awarded, have filed applications for, or have
been licensed under, numerous patents in the United States and other countries,
these patents may not fully protect our technology or competitive position.
Further, our competitors may apply for and obtain patents that will restrict our
ability to make and sell our products.

  Our competitors may intentionally infringe our patents.  Third parties may
also assert infringement claims against us in the future.  The defense and
prosecution of patent suits are both costly and time-consuming, even if the
outcome is favorable to us.  Outside the United States, such proceedings can be
extremely expensive, and their outcome very unpredictable.  An adverse outcome
in the defense of a patent suit could subject us to significant liabilities to
third parties or require us to license rights from third parties or to cease
selling our products.  We also rely on unpatented proprietary technology that
others may independently develop or otherwise obtain access to.  Our inability
to maintain the proprietary nature of our technologies could negatively affect
our revenues and earnings.

We may incur various tax liabilities that exceed our security for these
liabilities

  The former securityholders of the ADCS Group have agreed to indemnify us for
possible tax liabilities of the ADCS Group.  As security for these potential
liabilities, the former securityholders of the ADCS Group delivered into escrow
a portion of the shares of common stock they received in connection with our
acquisition of the ADCS Group.  We and the former securityholders of the ADCS
Group have divergent views on any potential exposures related to the various tax
matters for which there will be indemnification.  The former securityholders of
the ADCS Group believe that any exposure would be immaterial.  We believe that
any successful challenge to the tax matters is not probable.  While the possible
exposures are difficult to quantify, we believe that, regardless of the
probability that liabilities arise, the potential exposures could range from $0
to $22 million depending on the tax matter.  We have been notified by the
Internal Revenue Service of an assessment of $2.1 million for certain of these
tax matters.  Although we believe that this assessment is without merit and we
intend to vigorously defend our position on these tax matters, we cannot predict
whether we will be successful in defending against the assessment or the amount
of any final assessment against us.  We have agreed that minimum amounts must be
reached before we have a right to recover our losses.  The current value of the
shares held in escrow provides indemnity towards the upper range of the
potential exposures.  A possibility exists that the losses could exceed the
value of the shares held in escrow because the shares held in escrow are too few
in number or too low in value to adequately compensate us for these

                                      -8-
<PAGE>

losses. In any event, if we incur actual tax liabilities, our earnings, cash
flows and cash position will be negatively affected.

Our business could be negatively affected if we cannot define, develop and sell
new products

  We believe that our future success will depend, in part, upon our ability to
enhance our existing products and processes and to develop and commercialize new
products and processes.  We may not be able to improve our existing products and
process technologies or to develop and market new products and technologies that
will be cost-effective or introduced in a timely manner or accepted in the
marketplace.  Our failure to develop or introduce enhanced and new products and
processes in a timely manner may negatively affect our revenues and earnings.

We face numerous risks in manufacturing our products

  The manufacture of semiconductors and related materials and equipment involves
highly complex manufacturing processes. We have established manufacturing
facilities for many of our products, including semiconductor environmental
equipment, thin film materials, delivery systems, substrates and epitaxial
wafers. We have also established a facility to fabricate, test and assemble
semiconductor thin films, devices and circuits. Any prolonged disruption in our
manufacturing operations, whether due to technical or labor difficulties, delay
or inability to obtain sufficient quantities of production input or equipment,
destruction or damage to any facility or other reasons, could negatively impact
our ability to deliver products to customers. We are dependent upon a limited
number of suppliers to provide us with sufficient quantities of some materials
and equipment used in our processes so that we may be unable to obtain the
quantities of materials and equipment we require. To be financially successful,
we must manufacture our products in commercial quantities, at acceptable costs
and on a timely basis, which we may not be able to do. We have limited
experience in manufacturing certain of our products, and we may incur
significant start-up costs and unforeseen expenses in connection with attempts
to manufacture these products and expand our facilities.

Our business could be subject to product liability claims

  The manufacture and sale of our products, which include thin film and other
toxic materials, involve the risk of product liability claims.  In addition, a
failure of one of our products at a customer site could interrupt the business
operations of the customer. Our existing insurance coverage limits will not be
adequate to protect us from all liabilities that we might incur in connection
with the manufacture and sale of our products if a successful product liability
claim or series of product liability claims brought against us exceeds our
insurance coverage.

Our business is subject to substantial liabilities for failure to comply with
environmental regulations

  We use, generate and discharge toxic or otherwise hazardous chemicals and
wastes in our manufacturing, processing and research and development activities.
As a result, we are subject to a variety of governmental regulations related to
the storage, use and disposal of these materials. Our failure to comply with
present or future laws could result in fines or other liabilities being imposed
on us, suspension of production or a cessation of operations.

  The various premises we occupy, particularly the premises in Danbury,
Connecticut, may have been contaminated prior to occupancy.  We are not aware of
any environmental investigation or action by government agencies involving these
premises.  However, under federal and state statutes and regulations, a
government agency may seek to recover its response costs and/or require future
remedial measures from both operators and owners of property where releases of
hazardous substances have occurred or are ongoing.  The prior occupant of the
Danbury, Connecticut premises has agreed to indemnify us for remediation costs
in connection with any pre-existing, on-site contamination or environmental
condition.  However, this indemnification may not prove adequate to cover any
liability imposed on us related to the environmental condition of the premises
or the cost of defending an environmental action, either of which could be
substantial.

                                      -9-
<PAGE>

  Our activities may also result in our being subject to additional regulation.
Such regulations could require us to acquire significant additional equipment or
to incur other substantial expenses to comply with environmental laws.  Our
failure to control the use of hazardous substances could subject us to
substantial financial liabilities.

                                USE OF PROCEEDS

  We will not receive any proceeds from the sales of the shares by the selling
stockholders.  Also, we will bear most of the costs of registering the shares
covered by this prospectus.  Those costs include registration and filing fees
and fees and expenses of our counsel and accountants.

  However, the selling stockholders will be responsible for any underwriting
discounts and commissions or expenses incurred by the selling stockholders for
brokerage, accounting, tax or legal services.

                              SELLING STOCKHOLDERS

  The following table sets forth certain information known to us regarding the
beneficial ownership of  our common stock as of the date of this prospectus by
the selling stockholders.

<TABLE>
<CAPTION>
                                                Shares                                          Shares
                                           Beneficially Owned            Shares            Beneficially Owned
Selling Stockholders                      Prior to the Offering      Being Offered         After the Offering
- --------------------                      ---------------------      -------------         ------------------
<S>                                       <C>                        <C>                   <C>
High Street Capital, L.L.C.                      114,336               114,336
Michael A. Nemeroff                                2,382                 2,382                      0
KB Partners, L.L.C.                              116,990               116,990                      0
Andrew M. Denenberg as                           219,143               219,143                      0
 Trustee of Byron                                                                                   0
 Denenberg Grantor
 Retained Annuity Trust
 dated November 1, 1998
Byron A. Denenberg                               217,407               217,407
DJS Investment Partnership,                        9,528                 9,528                      0
 LLP                                                                                                0
John Cash                                          2,382                 2,382
David R. Chamberlin                               14,292                14,292                      0
 Revocable Trust dated                                                                              0
 11/11/75
David S. Connelly                                  2,382                 2,382
Robert J. Currey                                   4,764                 4,764                      0
Charles Dann                                       7,146                 7,146                      0
Bruce Denenberg IRA                                4,764                 4,764                      0
Howard W. Denenberg, M.D.                          2,382                 2,382                      0
Robert T. Geras                                    7,146                 7,146                      0
William R. Gurtin                                  9,528                 9,528                      0
Walter Haemmerli                                   4,764                 4,764                      0
Steven J. Hefter                                   9,528                 9,528                      0
Gerhard Hoog                                      17,436                17,436                      0
Richard E. Hulina                                  4,764                 4,764                      0
 Declaration of Trust dated                                                                         0
 May 6, 1994
Jaob Kiferbaum                                     4,764                 4,764
</TABLE>

                                      -10-
<PAGE>

<TABLE>
<CAPTION>
                                                Shares                                          Shares
                                           Beneficially Owned            Shares            Beneficially Owned
Selling Stockholders                      Prior to the Offering      Being Offered         After the Offering
- --------------------                      ---------------------      -------------         ------------------
<S>                                       <C>                        <C>                   <C>
Peter Koller                                       7,908                 7,908                      0
Edwin Kowalski                                       762                   762                      0
Donald A. Kurasch and                              7,146                 7,146                      0
 Mary J. Riordan as Joint                                                                           0
 Tenants
RL Partners                                        4,764                 4,764
Kenneth Leonard, Trustee,                          4,764                 4,764                      0
 Kenneth Leonard Pension                                                                            0
 Plan Trust
Lieberman Family LP                                9,528                 9,528                      0
John R. Obiala                                     2,382                 2,382                      0
Raymond J. Peacoe                                  2,382                 2,382                      0
Robert G. Quinn                                    4,764                 4,764                      0
The Riemer Family Trust                            4,764                 4,764                      0
 dated May 10, 1985
Nelson Rivera                                     17,436                17,436                      0
John R. Salerno                                    2,382                 2,382                      0
Daniel J. Savage, Trustee of                       4,764                 4,764                      0
 the Daniel J. Savage
 Declaration of Trust dated
 January 11, 1991
Centaur Partners V, L.P.                          11,910                11,910                      0
Richard Spinell                                    4,764                 4,764                      0
James Whiteside and                                2,382                 2,382                      0
 Michelle Whiteside as
 Joint Tenants
James J. Wiesen                                    2,382                 2,382                      0
IRA FBO John E.                                    2,382                 2,382                      0
 Wray/DLJSC as Custodian
Susan Donovan                                        838                   838                      0
Thomas Chatterton                                  2,062                 2,062                      0
Joseph Evans                                       1,429                 1,429                      0
Gordon Frank                                      20,265                20,265                      0
Dieter Kitzelmann, Ph.D.                          28,107                28,107                      0
Renate Merkel                                        762                   762                      0
Uwe Michalak                                       5,227                 5,227                      0
Peter Robertson                                    7,907                 7,907                      0
Telemind Corporation                               6,192                 6,192                      0
Mitsu Shida                                       12,233                12,233                      0
Ernesto Szombach                                   5,526                 5,526                      0
Gail Belian                                          762                   762                      0
Terrence Belian                                      762                   762                      0
Andrew M. Denenberg                                1,524                 1,524                      0
Gina M. Denenberg                                    762                   762                      0
Michael B. Denenberg                                 762                   762                      0
Nerrisa C. Denenberg                                 762                   762                      0
Timothy J. Denenberg                                 762                   762                      0
Amy Voss                                             762                   762                      0
Andrew Voss                                          762                   762                      0
Daniel B. Voss                                       762                   762                      0
Jared M. Voss                                        762                   762                      0
Paul Decker                                          762                   762                      0
</TABLE>

                                      -11-
<PAGE>

<TABLE>
<CAPTION>
                                                Shares                                          Shares
                                           Beneficially Owned            Shares            Beneficially Owned
Selling Stockholders                      Prior to the Offering      Being Offered         After the Offering
- --------------------                      ---------------------      -------------         ------------------
<S>                                       <C>                        <C>                   <C>
Denenberg 1998 Children's                         14,670                14,670                     0
 Trust dated November 26,
 1998
Keith D. Bank, as Trustee of                       4,764                 4,764                     0
 the Keith Bank
 Declaration of Trust dated
 December 14, 1994
Stephane Huynen                                  158,630               158,630                     0
Marleen Van Praag                                116,370               116,370                     0
Marc Huynen                                      158,630               158,630                     0
Therese Delsarte                                 116,370               116,370                     0

     TOTAL                                       873,025               873,025
</TABLE>


                              PLAN OF DISTRIBUTION

  The selling stockholders or their distributees or donees may sell the shares
in one or more transactions on the Nasdaq National Market or in negotiated
transactions, or both.  The selling stockholders may sell at market prices at
the time of sale, at prices related to the market price or at negotiated prices.
The selling stockholders may sell shares to or through broker-dealers, and the
broker-dealers may receive compensation in the form of discounts, concessions or
commissions from the selling stockholders and/or the purchasers of shares.  The
selling stockholders and any broker-dealers that participate in the sale of the
shares may be deemed to be "underwriters" within the meaning of Section 2(11) of
the Securities Act of 1933 and any commissions and any profit on the resale of
shares they receive may be deemed to be underwriting discounts and commissions
under the Securities Act.  We have agreed to indemnify the selling stockholders
against some liabilities, including liabilities under the Securities Act.

                                 LEGAL MATTERS

  Shipman & Goodwin LLP, of Hartford, Connecticut, will pass on the validity of
the shares offered by this prospectus.  As of the date of this prospectus,
lawyers employed at Shipman & Goodwin LLP owned 7,000 shares of our common
stock.

                                    EXPERTS

  The supplemental consolidated financial statements (including schedule) of
ATMI, Inc. appearing in the Company's Current Report on Form 8-K/A dated
November 29, 1999, incorporated by reference in this Registration Statement have
been audited by Ernst & Young LLP, independent auditors, as set forth in their
report thereon incorporated by reference elsewhere herein which, as to 1998 and
1997, is based in part on the reports of Arthur Andersen LLP, and as to 1998, is
based in part on the report of Rath, Anders, Dr. Wanner & Partner, and as to
1997, is based in part on the report of Deloitte & Touche LLP, and as to 1996,
is based in part on the reports of Deloitte & Touche LLP and Price Waterhouse
LLP, independent auditors. The supplemental consolidated financial statements
referred to above are incorporated herein by reference in reliance upon such
reports given on the authority of such firms as experts in accounting and
auditing.

  The supplemental consolidated financial statements (including schedule) of
ATMI, Inc. appearing in the Company's Current Report on Form 8-K/A dated May 31,
1999, incorporated by reference in this Registration Statement have been audited
by Ernst & Young LLP, independent auditors, as set forth in their report thereon
incorporated by reference elsewhere herein which, as to 1997, is based in part
on the report of Deloitte & Touche LLP, and as to 1996, is based in part on the
reports of Deloitte & Touche LLP and Price Waterhouse LLP, independent auditors.
The supplemental consolidated financial statements referred to above are
incorporated herein by reference in reliance upon such reports given on the
authority of such firms as experts in accounting and auditing.

                                      -12-
<PAGE>

     The consolidated financial statements (including schedule) of ATMI, Inc.
incorporated by reference in the Company's Annual Report (Form 10-K) for the
year ended December 31, 1998, have been audited by Ernst & Young LLP,
independent auditors, as set forth in their report thereon incorporated by
reference therein and incorporated herein by reference which, as to 1997, is
based in part on the report of Deloitte & Touche LLP, and as to 1996, is based
in part on the reports of Deloitte & Touche LLP and Price Waterhouse LLP,
independent auditors. Such consolidated financial statements are incorporated
herein by reference in reliance upon such reports given on the authority of such
firms as experts in accounting and auditing.

                                      -13-
<PAGE>

                                    PART II


                     INFORMATION NOT REQUIRED IN PROSPECTUS


Item 14.  Other Expenses of Issuance and Distribution

     The following table sets forth the estimated expenses, other than broker-
dealer discounts and commissions, payable by the registrant in connection with
the sale and distribution of the shares of Common Stock registered hereby.  The
registrant is paying all of the expenses incurred on behalf of the selling
stockholders (other than discounts and commissions).


     SEC registration fee......................   $12,541
     Legal fees and expenses...................     5,000
     Accounting fees and expenses..............     5,000
     Miscellaneous.............................     5,000
                                                    -----
          Total expenses.......................   $27,541


Item 15.  Indemnification of Directors and Officers

     The registrant's Certificate of Incorporation provides that the personal
liability of the directors of the registrant shall be limited to the fullest
extent permitted by the provisions of Section 102(b)(7) of the General
Corporation Law of the State of Delaware (the "DGCL").  Section 102(b)(7) of the
DGCL generally provides that no director shall be liable personally to the
registrant or its stockholders for monetary damages for breach of fiduciary duty
as a director; however, the Certificate of Incorporation does not eliminate the
liability of a director for (i) any breach of the director's duty of loyalty to
the registrant or its stockholders; (ii) acts or omissions not in good faith or
that involve intentional misconduct or a knowing violation of law; (iii) acts or
omissions in respect of certain unlawful dividend payments or stock redemptions
or repurchases; or (iv) any transaction from which such director derives
improper personal benefit.  The effect of this provision is to eliminate the
rights of the registrant and its stockholders (through stockholders' derivatives
suits on behalf of the registrant) to recover monetary damages against a
director for breach of her or his fiduciary duty of care as a director
(including breaches resulting from negligent or grossly negligent behavior)
except in the situations described in clauses (i) through (iv) above.  The
limitations summarized above, however, do not affect the ability of the
registrant or its stockholders to seek nonmonetary remedies, such as an
injunction or rescission, against a director for breach of her or his fiduciary
duty.

     In addition, the registrant's Certificate of Incorporation and Bylaws
provide that the registrant shall, to the fullest extent permitted by Section
145 of the DGCL, indemnify all persons whom it may indemnify pursuant to Section
145 of the DGCL. Section 145 of the DGCL permits a company to indemnify an
officer or director who was or is a party or is threatened to be made a party to
any proceeding because of his or her position, if the officer or director acted
in good faith and in a manner he or she reasonably believed to be in or not
opposed to the best interests of the company and, with respect to any criminal
action or proceeding, had no reasonable cause to believe his or her conduct was
unlawful. Insofar as indemnification for liabilities arising under the
Securities Act of 1933, as amended (the "Securities Act"), may be permitted to
directors, officers, or persons controlling the registrant pursuant to the
foregoing provisions, the registrant has been informed that in the opinion of
the Securities and Exchange Commission (the "Commission"), such indemnification
is against public policy as expressed in the Securities Act and is therefore
unenforceable.

     The registrant maintains insurance for officers and directors against
certain liabilities, including liabilities under the Securities Act. The effect
of this insurance is to indemnify any officer or director of the registrant
against expenses, including, without limitation, attorneys' fees, judgments,
fines and amounts paid in settlement, incurred by an officer or director upon a
determination that such person acted in good faith. The premiums for such
insurance are paid by the registrant.

                                      II-1
<PAGE>

Item 16.  Exhibits

     The following is a list of exhibits filed as a part of this registration
statement or incorporated by reference herein:

Exhibit No.         Description
- -----------         -----------

   4.1(a)      Certificate of Incorporation of ATMI (Exhibit 3.01 to ATMI's
               Registration Statement on Form S-4, filed September 10, 1997,
               Registration No. 333-35323 (the "1997 Form S-4 Registration
               Statement")). (1)

   4.1(b)      Certificate of Amendment to Certificate of Incorporation (Exhibit
               4.1(b) to ATMI's Post-Effective Amendment No. 1 to Registration
               Statement on Form S-8, filed October 10, 1997, Registration No.
               33-77060). (1)

   4.1(c)      Certificate of Amendment to Certificate of Incorporation (Exhibit
               3.01(c) to ATMI's Registration Statement on Form S-4,
               Registration No. 333-51333). (1)

   4.2         Bylaws of ATMI (Exhibit 3.02 to the 1997 Form S-4 Registration
               Statement). (1)

   5.1         Opinion and Consent of Shipman & Goodwin LLP as to the legality
               of the shares to be registered. (2)

  23.1         Consent of Shipman & Goodwin LLP, included in opinion filed as
               Exhibit 5.1. (2)

  23.2         Consent of Ernst & Young LLP. (2)

  23.3         Consent of PricewaterhouseCoopers LLP. (2)

  23.4         Consent of Deloitte & Touche LLP. (2)

  23.5         Consent of Arthur Andersen LLP. (2)

  23.6         Consent of Rath, Anders, Dr. Wanner & Partner. (2)

  24.1         Power of Attorney, included in signature page of this
               Registration Statement. (2)

__________________
(1)  Incorporated by reference.

(2)  Filed herewith.


Item 17.  Undertakings

     The undersigned registrant hereby undertakes:

         (1) To file, during any period in which offers or sales are being made,
         a post-effective amendment to this Registration Statement to include
         any material information with respect to the plan of distribution not
         previously disclosed in the registration statement or any material
         change to such information in the registration statement.

         (2) That, for the purpose of determining any liability under the
         Securities Act, each such post-effective amendment shall be deemed to
         be a new registration statement relating to the securities offered
         therein, and the offering of the securities at that time shall be
         deemed to be the initial bona fide offering thereof.

                                      II-2
<PAGE>

         (3) To remove from registration by means of a post-effective amendment
         any of the securities being registered which remain unsold at the
         termination of the offering.

     The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to Section 15(d) of the
Securities Exchange Act of 1934) that is incorporated by reference in the
Registration Statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

     Insofar as indemnification for liabilities arising under the Securities Act
may be permitted to the 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 Commission such indemnification is
against public policy as expressed in the Securities 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 or 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 question whether such indemnification by it is against public
policy as expressed in the Securities Act and will be governed by the final
adjudication of such issue.

     The undersigned registrant hereby undertakes that:


         (1) For purposes of determining any liability under the Securities Act,
         the information omitted from the form of prospectus filed as part of
         this registration statement in reliance upon Rule 430A and contained in
         a form of prospects 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, each post-effective amendment that contains a form of prospectus
         shall be deemed to be a new registration statement relating to the
         securities offered therein, and the offering of such securities at that
         time shall be deemed to be the initial bona fide offering thereof.

                                      II-3
<PAGE>

                                   SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Danbury, State of Connecticut, on January 13, 2000.


                          ATMI, INC.


                          By:  /s/ Eugene G. Banucci
                               ---------------------
                               Eugene G. Banucci, President,
                               Chief Executive Officer and Chairman of the Board

                               POWER OF ATTORNEY

     Know All Persons by These Presents, that each person whose signature
appears below constitutes and appoints Eugene G. Banucci and Daniel P. Sharkey
and each of them, his or her true and lawful attorneys-in-fact and agents, with
full power of substitution and resubstitution for him or her and in his or her
name, place and stead, in any and all capacities to sign any and all amendments
(including, without limitation, post-effective amendments and any amendment or
amendments or registration statement increasing the amount of securities for
which registration is being sought) to this registration statement, and to file
the same, with all exhibits thereto, and other documents in connection
therewith, with full power and authority to do and perform each and every act
and thing requisite or necessary to be done in and about the premises, as fully
to all intents and purposes as he or she might or could do in person, hereby
ratifying and confirming all that said attorneys-in-fact and agents or any of
them, of their or his substitute or substitutes, may lawfully do or cause to be
done by virtue hereof.


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


     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>
           Signature                                            Title                              Date
           ---------                                            -----                              ----
<S>                                                <C>                                        <C>
          /s/ Eugene G. Banucci                    President, Chief Executive Officer,       January 13, 2000
- ------------------------------------------------    Chairman of the Board and Director
              Eugene G. Banucci                        (principal executive officer)

          /s/ Daniel P. Sharkey                    Vice President, Treasurer and Chief       January 13, 2000
- ------------------------------------------------       Financial Officer (principal
              Daniel P. Sharkey                      financial and accounting officer)

          /s/ Mark A. Adley                                      Director                    January 13, 2000
- ------------------------------------------------
              Mark A. Adley

          /s/ Robert S. Hillas                                   Director                    January 13, 2000
- ------------------------------------------------
              Robert S. Hillas

          /s/ Kam Law                                            Director                    January 13, 2000
- ------------------------------------------------
              Kam Law
</TABLE>

                                     II-4
<PAGE>

<TABLE>
<CAPTION>
                Signature                                         Title                           Date
                ---------                                         -----                           ----
<S>                                                             <C>                          <C>
          /s/ Stephen H. Mahle                                   Director                    January 13, 2000
- ------------------------------------------------
              Stephen H. Mahle

          /s/ Stephen H. Siegele                                 Director                    January 13, 2000
- ------------------------------------------------
              Stephen H. Siegele
</TABLE>

                                     II-5

<PAGE>

                                 EXHIBIT INDEX


Exhibit No.                      Description
- -----------                      -----------

     4.1(a)    Certificate of Incorporation of Registrant (Exhibit 3.01 to
               ATMI's Registration Statement on Form S-4, filed September 10,
               1997, Registration No. 333-35323 (the "1997 Form S-4 Registration
               Statement")). (1)

     4.1(b)    Certificate of Amendment to Certificate of Incorporation (Exhibit
               4.1(b) to ATMI's Post-Effective Amendment No. 1 to Registration
               Statement on Form S-8, filed October 10, 1997, Registration No.
               33-77060). (1)

     4.1(c)    Certificate of Amendment to Certificate of Incorporation (Exhibit
               3.01(c) to ATMI's Registration Statement on Form S-4,
               Registration No. 333-51333). (1)

     4.2       Bylaws of ATMI (Exhibit 3.02 to the 1997 Form S-4 Registration
               Statement). (1)

     5.1       Opinion and Consent of Shipman & Goodwin LLP as to the legality
               of the shares to be registered. (2)

    23.1       Consent of Shipman & Goodwin LLP, included in opinion filed as
               Exhibit 5.1. (2)

    23.2       Consent of Ernst & Young LLP. (2)

    23.3       Consent of PricewaterhouseCoopers LLP. (2)

    23.4       Consent of Deloitte & Touche LLP. (2)

    23.5       Consent of Arthur Andersen LLP. (2)

    23.6       Consent of Rath, Anders, Dr. Wanner & Partner. (2)

    24.1       Power of Attorney, included in signature page of this
               Registration Statement. (2)


    _______________
 (1)  Incorporated by reference.
 (2)  Filed herewith.



                                      E-1

<PAGE>

                                                                    Exhibit 5.01

Shipman & Goodwin LLP                        One American Row
 Counselors at Law                           Hartford, CT 06103
                                             TEL: (860) 251-5000


                              January 13, 2000


Board of Directors
ATMI, Inc.
7 Commerce Drive
Danbury, Connecticut 06810

     Re:  Registration Statement on Form S-3
          ----------------------------------

Ladies and Gentlemen:

     We have served as counsel for ATMI, Inc., a Delaware corporation (the
"Company"), in connection with the proposed sale by certain stockholders of a
maximum of 1,543,282 shares of Common Stock of the Company (the "Shares")
pursuant to a Registration Statement on Form S-3 to be filed with the Securities
and Exchange Commission (the "Registration Statement").

     As counsel to the Company, we have examined the Registration Statement,
including the Prospectus contained therein, and such other documents as we have
deemed necessary or appropriate in order to express the opinion set forth below.
In connection with our opinion hereinafter given, we have examined and relied
upon originals, or copies, certified or otherwise, identified to our
satisfaction, of such agreements, documents, certificates and other statements
of government officials, corporate officers and representatives and other
documents as we have deemed relevant and necessary as a basis for such opinion.
In such examination, we have assumed the genuineness of all signatures and the
authenticity of all documents submitted to us as originals and the conformity
with the original documents of documents submitted to us as copies.

     Based upon the foregoing, we are of the opinion that the Shares have been
duly authorized and legally issued and are fully paid and non-assessable.

     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the use of our name under the heading "Legal
Matters" in the Prospectus included in the Registration Statement.  In giving
this consent, we do not thereby admit that we come within the category of
persons whose consent is required under Section 7 of the Securities Act or the
rules and regulations of the Securities and Exchange Commission.

                              Very truly yours,


                              /s/ SHIPMAN & GOODWIN LLP

<PAGE>

                                                                    Exhibit 23.2


                        CONSENT OF INDEPENDENT AUDITORS

We consent to the reference to our firm under the caption "Experts" in the
Registration Statement (Form S-3) and related prospectus of ATMI, Inc. for the
registration of 1,543,282 shares of its common stock and to the incorporation by
reference therein of our report dated February 8, 1999 with respect to the
consolidated financial statements and schedule of ATMI included in the Annual
Report on Form 10-K and our reports dated June 29, 1999 and January 11, 2000,
with respect to the supplemental consolidated financial statements and schedule
of ATMI included in its Current Reports dated May 31, 1999 and November 29, 1999
(Form 8-K/A) for the year ended December 31, 1998, filed with the Securities and
Exchange Commission.


/s/ Ernst & Young LLP
Stamford, Connecticut
January 11, 2000

<PAGE>

                                                                    Exhibit 23.3



                       Consent of Independent Accountants

We hereby consent to the incorporation by reference in the Registration
Statement on Form S-3 of ATMI, Inc. of our report dated May 17, 1997, except for
the last paragraph of Note 3 which is as of July 29, 1997 and the last paragraph
of Note 6 which is as of December 18, 1997, pertaining to the combined financial
statements of Lawrence Semiconductor Laboratories, Inc. and Affiliate as of and
for the year ended December 31, 1996 appearing in ATMI, Inc.'s Annual Report on
Form 10-K for the year ended December 31, 1998. It should be noted, however,
that such financial statements are not included separately in ATMI, Inc.'s Form
10-K. We also consent to the application of our report to the Financial
Statement Schedule for the year ended December 31, 1996 appearing in the Form
10-K when such schedule is read in conjunction with the financial statements
referred to in our report. The audit referred to in our report also included
this schedule.


/s/ PricewaterhouseCoopers LLP

PricewaterhouseCoopers LLP
Phoenix, Arizona
January 11, 2000

<PAGE>

                                                                    Exhibit 23.4



INDEPENDENT AUDITORS' CONSENT

We consent to the incorporation by reference in this Registration Statement of
ATMI, Inc. on Form S-3 of our report dated May 1, 1998 relating to NOW
Technologies, Inc. and Subsidiaries appearing in ATMI, Inc.'s Current Report on
Form 8-K/A dated November 29, 1999 and in the Annual Report on Form 10-K of
ATMI, Inc. for the year ended December 31, 1998.


/s/ Deloitte & Touche LLP

Minneapolis, Minnesota
January 11, 2000

<PAGE>

                                                                    Exhibit 23.5

                       Consent of Independent Accountants


As independent public accountants, we hereby consent to the incorporation by
reference in this registration statement of our reports dated March 11, 1998 and
May 28, 1999 included in ATMI, Inc.'s Form 8K/A dated November 29, 1999 and all
references to our Firm included in this registration statement.  We also consent
to the application of our reports to the schedule included in that 8K/A labeled
"Valuation and Qualification Accounts" as of the dates and period covered by our
reports.

/s/ Arthur Andersen LLP

Chicago, Illinois
January 11, 2000

<PAGE>

                                                                    Exhibit 23.6

                        CONSENT OF INDEPENDENT AUDITORS


As independent public accountants, we hereby consent to the incorporation by
reference in this registration statement of our reports dated January 22, 1999
included in ATMI, Inc.'s Form 8K/A dated November 29, 1999 and all references to
our Firm included in this registration statement. We also consent to the
application of our reports to the schedule included in that 8K/A labeled.
Valuation and Qualifying Accounts as of the dates and period covered by our
reports.


Rath, Anders, Dr. Wanner & Partner
Munich, January 11, 2000


              /s/ Kabisch                     /s/ Metzler
              Dipl.-Kfm. Kabisch              Dipl.-Kfm. Metzler
              Vereidigter Buchprufer          Wirtschaftsprufer


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