S&T BANCORP INC
SC 13E4, 1998-01-20
STATE COMMERCIAL BANKS
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<PAGE>

 
 
   AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JANUARY 20, 1998
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
 
                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549

                                 ------------
 
                                SCHEDULE 13E-4
                         ISSUER TENDER OFFER STATEMENT
     (Pursuant to Section 13(e)(1) of the Securities Exchange Act of 1934)
 
                               S&T BANCORP, INC.
                               (Name of issuer)
 
                               S&T BANCORP, INC.
                     (Name of Person(s) Filing Statement)
 
                    COMMON STOCK, $2.50 PAR VALUE PER SHARE
                        (Title of Class of Securities)
 
                                   783859101
                     (CUSIP Number of Class of Securities)
 
                                ROBERT E. ROUT
                            CHIEF FINANCIAL OFFICER
                               S&T BANCORP, INC.
                            800 PHILADELPHIA STREET
                          INDIANA, PENNSYLVANIA 15701
                                (412) 349-1800
      (Name, Address and Telephone Number of Person Authorized to Receive
    Notices and Communications on Behalf of the Person(s) Filing Statement)
 
                                  COPIES TO:
                               RICHARD E. BALTZ
                                ARNOLD & PORTER
                           555 TWELFTH STREET, N.W.
                            WASHINGTON, D.C. 20004
                                (202) 942-5124
                             (202) 942-5999 (FAX)

                                 ------------
 
                               JANUARY 20, 1998
    (Date Tender Offer First Published, Sent or Given to Security Holders)

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

                           CALCULATION OF FILING FEE
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<TABLE>
<CAPTION>
                       TRANSACTION VALUATION*              AMOUNT OF FILING FEE*
- --------------------------------------------------------------------------------
<S>                   <C>                                  <C>
                            $73,500,000                           $14,700
</TABLE>
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
  * Calculated solely for the purpose of determining the filing fee, based upon
    the purchase of 1,500,000 shares at the maximum tender offer price of $49.00
    per share.
 
[_] Check box if any of the fee is offset as provided by Rule 0-11(a)(2) and
    identify the filing with which the offsetting fee was previously paid.
    Identify the previous filing by registration statement number, or the Form
    or Schedule and the date of its filing.

                                 ------------
  
          Amount Previously Paid: N/A               Filing Party: N/A
 
          Form or Registration No.: N/A             Date Filed: N/A
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
<PAGE>
 
ITEM 1. SECURITY AND ISSUER.
 
  (a) The issuer of the securities to which this Schedule 13E-4 relates is S&T
Bancorp, Inc., a Pennsylvania corporation (the "Company"), and the address of
its principal executive office, and its mailing address, is 800 Philadelphia
Street, Indiana, Pennsylvania 15701.
 
  (b) This Schedule 13E-4 relates to the offer by the Company to purchase up
to 1,500,000 shares (or such lesser number of shares as are properly tendered)
of its common stock, $2.50 par value per share (the "Shares"), 14,143,655 of
which Shares were outstanding as of January 15, 1998, at prices not less than
$46.00 nor in excess of $49.00 per Share in cash, upon the terms and subject
to the conditions set forth in the Offer to Purchase, dated January 20, 1998
(the "Offer to Purchase"), and in the related Letter of Transmittal (which
together constitute the "Offer"), copies of which are attached as Exhibits
(a)(1) and (a)(2), respectively, and incorporated herein by reference.
Employees, officers and directors of the Company may participate in the Offer
on the same basis as the Company's other shareholders. The information set
forth in "Introduction", "Section 1--Number of Shares; Proration" and "Section
11--Interest of Directors and Executive Officers; Transactions and
Arrangements Concerning Shares" of the Offer to Purchase is incorporated
herein by reference.
 
  (c) The information set forth in "Introduction" and "Section 7--Price Range
of Shares; Dividends" of the Offer to Purchase is incorporated herein by
reference.
 
  (d) Not applicable.
 
ITEM 2. SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION.
 
  (a)-(b) The information set forth in "Section 10--Source and Amount of
Funds" of the Offer to Purchase is incorporated herein by reference.
 
ITEM 3. PURPOSE OF THE TENDER OFFER AND PLANS OR PROPOSALS OF THE ISSUER OR
AFFILIATE.
 
  (a)-(j) The information set forth in "Introduction" and "Section 10--Source
and Amount of Funds," "Section 8--Purpose of the Offer; Certain Effects of the
Offer," "Section 11--Interest of Directors and Executive Officers;
Transactions and Arrangements Concerning Shares" and "Section 12--Effects of
the Offer on the Market for Shares; Registration under the Exchange Act" of
the Offer to Purchase is incorporated herein by reference.
 
ITEM 4. INTEREST IN SECURITIES OF THE ISSUER.
 
  The information set forth in "Section 11--Interest of Directors and
Officers; Transactions and Arrangements Concerning Shares" and "Schedule A" of
the Offer to Purchase is incorporated herein by reference.
 
ITEM 5. CONTRACTS, ARRANGEMENTS, UNDERSTANDINGS OR RELATIONSHIPS WITH RESPECT
       TO THE ISSUER'S SECURITIES.
 
  The information set forth in "Introduction" and "Section 10--Source and
Amount of Funds," "Section 8--Purpose of the Offer; Certain Effects of the
Offer," and "Section 11--Interest of Directors and Executive Officers;
Transactions and Arrangements Concerning Shares" of the Offer to Purchase is
incorporated herein by reference.
 
ITEM 6. PERSONS RETAINED, EMPLOYED, OR TO BE COMPENSATED.
 
  The information set forth in "Introduction" and "Section 16--Fees and
Expenses" of the Offer to Purchase is incorporated herein by reference.
 
ITEM 7. FINANCIAL CONDITION.
 
  (a)-(b) The information set forth in "Section 9--Certain Information
Concerning the Company" of the Offer to Purchase is incorporated herein by
reference.
<PAGE>
 
ITEM 8. ADDITIONAL INFORMATION.
 
  (a) Not applicable.
 
  (b) The information set forth in "Section 13--Certain Legal Matters;
      Regulatory Approvals" of the Offer to Purchase is incorporated herein
      by reference.
 
  (c) The information set forth in "Section 12--Effects of the Offer on the
      Market for Shares; Registration under the Exchange Act" of the Offer to
      Purchase is incorporated herein by reference.
 
  (d) Not applicable.
 
  (e) The information set forth in the Offer to Purchase and Letter of
      Transmittal is incorporated herein by reference.
 
ITEM 9. MATERIAL TO BE FILED AS EXHIBITS.
 
  (a)(1)Form of Offer to Purchase, dated January 20, 1998.
 
     (2) Form of Letter of Transmittal (including Certification of Taxpayer
         Identification Number on Form W-9).
 
     (3) Form of Notice of Guaranteed Delivery.
  
     (4) Form of Letter to Brokers, Dealers, Commercial Banks, Trust
         Companies and Other Nominees.
 
     (5) Form of Letter to Clients for Use by Brokers, Dealers, Commercial
         Banks, Trust Companies and Other Nominees (including the
         Instruction Form).
 
     (6) Form of Letter to Shareholders of the Company, dated January 20,
         1998, from James C. Miller, President and Chief Executive Officer
         of the Company.
 
     (7) Form of Letter to Participants in Thrift Plan for Employees of S&T
         Bancorp, Inc.
 
     (8) Form of Memorandum, dated January 20, 1998 to the Company's
         employees.
 
     (9) Form of Question and Answer Brochure.
 
    (10) Text of Press Release issued by the Company, dated January 20, 1998.
 
    (11) Text of Press Announcement to be published in local and regional
         newspapers on January 20, 1998.
 
    (12) Text of Press Release Announcing 1997 Earnings issued by the
         Company, dated January 20, 1998.
 
  (b) Not applicable.
 
  (c) Not applicable.
 
  (d) Not applicable.
 
  (e) Not applicable.
 
  (f) Not applicable.
<PAGE>
 
                                   SIGNATURE
 
  After due inquiry and to the best of my knowledge and belief, I certify that
the information set forth in this Schedule 13E-4 is true, complete and
correct.
 
January 20, 1998                          S&T BANCORP, INC.
 
                                          By: /s/ James C. Miller
                                          -------------------------------------
                                          James C. Miller
                                          President and Chief Executive
                                          Officer
<PAGE>
 
                                                                  Exhibit(a)(1)
                               S&T BANCORP, INC.
 
                          OFFER TO PURCHASE FOR CASH
 
                         UP TO 1,500,000 SHARES OF ITS
                    COMMON STOCK, PAR VALUE $2.50 PER SHARE
 
                   AT A PURCHASE PRICE NOT LESS THAN $46.00
                       NOR IN EXCESS OF $49.00 PER SHARE
                                 ------------
 
           THE OFFER, PRORATION PERIOD AND WITHDRAWAL RIGHTS EXPIRE
               AT 5:00 P.M., EASTERN TIME, ON FEBRUARY 18, 1998
                         UNLESS THE OFFER IS EXTENDED.
                                 ------------
 
  S&T Bancorp, Inc., a Pennsylvania corporation (the "Company"), invites its
shareholders to tender shares of its Common Stock, par value $2.50 per share
(the "Shares"), at prices, net to the seller in cash, without interest
thereon, not less than $46.00 nor in excess of $49.00 per Share specified by
such tendering shareholders, upon the terms and subject to the conditions set
forth herein and in the related Letter of Transmittal (which together
constitute the "Offer"). The Company will determine a single per Share price
(not less than $46.00 nor in excess of $49.00 per Share) that it will pay for
the Shares validly tendered pursuant to the Offer and not withdrawn (the
"Purchase Price"), taking into consideration the number of Shares so tendered
and the prices specified by the tendering shareholders. The Company will
select the Purchase Price that will allow it to purchase 1,500,000 Shares (or
such lesser number of Shares as are validly tendered and not withdrawn at
prices not less than $46.00 nor in excess of $49.00 per Share) pursuant to the
Offer. The Company will purchase all Shares validly tendered at prices at or
below the Purchase Price and not withdrawn on or prior to the Expiration Date
(as defined in Section 1), upon the terms and subject to the conditions of the
Offer, including the provisions thereof relating to proration described
herein. The Purchase Price will be paid in cash, net to the seller, without
interest thereon, with respect to all Shares purchased. All Shares tendered at
prices in excess of the Purchase Price and Shares not purchased because of
proration will be returned. Shareholders must complete the section of the
Letter of Transmittal relating to the price at which they are tendering Shares
in order to validly tender Shares.
                                 ------------
 
  THE OFFER IS NOT CONDITIONED UPON ANY MINIMUM NUMBER OF SHARES BEING
TENDERED. THE OFFER IS, HOWEVER, SUBJECT TO OTHER CONDITIONS. SEE SECTION 6.
                                 ------------
 
                                   IMPORTANT
 
  Any shareholder desiring to tender all or any portion of his or her Shares
should either (1) complete and sign the Letter of Transmittal or a facsimile
thereof in accordance with the instructions in the Letter of Transmittal, mail
or deliver it and any other required documents to ChaseMellon Shareholder
Services, L.L.C. (the "Depositary"), and either mail or deliver the
certificates representing Shares to be tendered to the Depositary along with
the Letter of Transmittal or deliver such Shares pursuant to the procedure for
book-entry transfer set forth in Section 3 or (2) request his or her broker,
dealer, commercial bank, trust company or nominee to effect the transaction
for him or her. A shareholder whose Shares are registered in the name of a
broker, dealer, commercial bank, trust company or nominee must contact such
broker, dealer, commercial bank, trust company or nominee if he or she desires
to tender such Shares. Shareholders who desire to tender Shares and whose
certificates for such Shares are not immediately available or who cannot
comply with the procedure for book-entry transfer by the expiration of the
Offer must tender such Shares by following the procedures for guaranteed
delivery set forth under "Section 3--Procedure for Tendering Shares."
SHAREHOLDERS MUST PROPERLY COMPLETE THE LETTER OF TRANSMITTAL, INCLUDING THE
SECTION OF THE LETTER OF TRANSMITTAL RELATING TO THE PRICE AT WHICH THEY ARE
TENDERING SHARES, IN ORDER TO EFFECT A VALID TENDER OF THEIR SHARES.
 
 THE BOARD OF  DIRECTORS OF THE  COMPANY HAS UNANIMOUSLY  APPROVED THE OFFER.
  NEITHER THE COMPANY NOR ITS BOARD OF DIRECTORS MAKES ANY RECOMMENDATION TO
   ANY  SHAREHOLDER  AS TO  WHETHER  TO  TENDER  ALL  OR ANY  SHARES.  EACH
    SHAREHOLDER MUST MAKE HIS OR HER  OWN DECISION AS TO WHETHER TO TENDER
     SHARES AND, IF SO,  HOW MANY TO TENDER AND AT WHAT PRICE. EMPLOYEES,
       DIRECTORS AND EXECUTIVE OFFICERS MAY PARTICIPATE IN THE OFFER  ON
        THE SAME BASIS AS THE COMPANY'S OTHER SHAREHOLDERS.
 
                     The Dealer Manager for the Offer is:
                         KEEFE, BRUYETTE & WOODS, INC.
 
                    The Information Agent for the Offer is:
                    CORPORATE INVESTOR COMMUNICATIONS, INC.
 
            The date of this Offer to Purchase is January 20, 1998
<PAGE>
 
  As of January 15, 1998, the Company had issued and outstanding 14,143,655
Shares, and had reserved 542,000 Shares for issuance upon exercise of
currently exercisable stock options under the Company's Incentive Stock Plan.
The 1,500,000 Shares that the Company is offering to purchase pursuant to the
Offer represent approximately 11% of the Shares then outstanding. The Shares
are traded on the Nasdaq National Market. The Shares trade under the symbol
"STBA." On January 15, 1998, the closing price of the Shares as reported on
the Nasdaq National Market was $43.75 per Share. SHAREHOLDERS ARE URGED TO
OBTAIN CURRENT MARKET QUOTATIONS FOR THE SHARES.
 
  TO TENDER SHARES PROPERLY, SHAREHOLDERS MUST COMPLETE THE SECTION OF THE
LETTER OF TRANSMITTAL RELATING TO THE PRICE AT WHICH THEY ARE TENDERING
SHARES.
 
  Questions or requests for assistance or for additional copies of this Offer
to Purchase, the Letter of Transmittal or other tender offer materials may be
directed to the Information Agent or the Dealer Manager at their respective
addresses and telephone numbers set forth on the back cover of this Offer to
Purchase, and such copies will be furnished promptly at the Company's expense.
Shareholders may also contact their local broker, dealer, commercial bank or
trust company for assistance concerning the Offer.
 
  NO PERSON HAS BEEN AUTHORIZED TO MAKE ANY RECOMMENDATION ON BEHALF OF THE
COMPANY AS TO WHETHER SHAREHOLDERS SHOULD TENDER SHARES PURSUANT TO THE OFFER.
NO PERSON HAS BEEN AUTHORIZED TO GIVE ANY INFORMATION OR TO MAKE ANY
REPRESENTATIONS IN CONNECTION WITH THE OFFER OTHER THAN THOSE CONTAINED HEREIN
OR IN THE RELATED LETTER OF TRANSMITTAL. IF GIVEN OR MADE, SUCH RECOMMENDATION
AND SUCH OTHER INFORMATION AND REPRESENTATIONS MUST NOT BE RELIED UPON AS
HAVING BEEN AUTHORIZED BY THE COMPANY.
<PAGE>
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
 SECTION                                                                   PAGE
 -------                                                                   ----
 <C>     <S>                                                               <C>
         INTRODUCTION....................................................    1
    1.   NUMBER OF SHARES; PRORATION.....................................    2
    2.   TENDERS BY HOLDERS OF FEWER THAN 100 SHARES.....................    3
    3.   PROCEDURE FOR TENDERING SHARES..................................    4
    4.   WITHDRAWAL RIGHTS...............................................    6
    5.   ACCEPTANCE FOR PAYMENT OF SHARES AND PAYMENT OF PURCHASE PRICE..    7
    6.   CERTAIN CONDITIONS OF THE OFFER.................................    8
    7.   PRICE RANGE OF SHARES; DIVIDENDS................................   10
    8.   PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER..............   10
    9.   CERTAIN INFORMATION CONCERNING THE COMPANY......................   11
   10.   SOURCE AND AMOUNT OF FUNDS......................................   21
   11.   INTEREST OF DIRECTORS AND EXECUTIVE OFFICERS; TRANSACTIONS AND
         ARRANGEMENTS CONCERNING SHARES..................................   21
   12.   EFFECTS OF THE OFFER ON THE MARKET FOR SHARES; REGISTRATION
         UNDER THE EXCHANGE ACT..........................................   22
   13.   CERTAIN LEGAL MATTERS; REGULATORY APPROVALS.....................   23
   14.   CERTAIN FEDERAL INCOME TAX CONSEQUENCES.........................   23
   15.   EXTENSION OF TENDER PERIOD; TERMINATION; AMENDMENTS.............   26
   16.   FEES AND EXPENSES...............................................   27
   17.   ADDITIONAL INFORMATION..........................................   27
   18.   MISCELLANEOUS...................................................   27
</TABLE>
<PAGE>
 
To the Holders of Shares of Common Stock of
S&T Bancorp, Inc.:
 
                                 INTRODUCTION
 
  S&T Bancorp, Inc., a Pennsylvania corporation (the "Company" or "S&T"),
invites its shareholders to tender shares of its Common Stock, par value $2.50
per share (the "Shares"), at a price, net to the seller in cash, without
interest thereon, not less than $46.00 nor in excess of $49.00 per Share
specified by such tendering shareholders, upon the terms and subject to the
conditions set forth herein and in the related Letter of Transmittal (which
together constitute the "Offer").
 
  The Company will determine a single per Share price (not less than $46.00
nor in excess of $49.00 per Share) that it will pay for the Shares validly
tendered pursuant to the Offer and not withdrawn (the "Purchase Price"),
taking into account the number of Shares so tendered and the prices specified
by tendering shareholders. The Company will select the Purchase Price that
will allow it to purchase 1,500,000 Shares (or such lesser number of Shares as
is validly tendered and not withdrawn at prices not less than $46.00 nor in
excess of $49.00 per Share) pursuant to the Offer. The Company will purchase
all Shares validly tendered at prices at or below the Purchase Price and not
withdrawn on or prior to the Expiration Date (as defined in Section 1), upon
the terms and subject to the conditions of the Offer, including the provisions
relating to proration described below. The Purchase Price will be paid in
cash, net to the seller, without interest thereon, with respect to all Shares
purchased. Shares tendered at prices in excess of the Purchase Price and
Shares not purchased because of proration will be returned.
 
  THE OFFER IS NOT CONDITIONED UPON ANY MINIMUM NUMBER OF SHARES BEING
TENDERED. THE OFFER IS, HOWEVER, SUBJECT TO CERTAIN OTHER CONDITIONS. SEE
SECTION 6.
 
  If more than 1,500,000 Shares have been validly tendered at or below the
Purchase Price and not withdrawn on or prior to the Expiration Date, the
Company will purchase Shares first from shareholders who owned beneficially as
of the close of business on January 15, 1998, and continue to own beneficially
as of the Expiration Date, an aggregate of fewer than 100 Shares who properly
tender all their Shares at or below the Purchase Price, and then on a pro rata
basis from all other shareholders who validly tender Shares at or below the
Purchase Price. See Sections 1 and 2. Tendering shareholders will not be
obligated to pay brokerage commissions, solicitation fees or, subject to the
Instructions to the Letter of Transmittal, stock transfer taxes on the
purchase of Shares by the Company. The Company will pay the expenses of Keefe,
Bruyette & Woods, Inc. (the "Dealer Manager"), ChaseMellon Shareholder
Services, L.L.C. (the "Depositary") and Corporate Investor Communications,
Inc. (the "Information Agent") incurred in connection with the Offer. See
Section 16. ANY TENDERING SHAREHOLDER OR OTHER PAYEE WHO FAILS TO COMPLETE AND
SIGN THE SUBSTITUTE FORM W-9 THAT IS INCLUDED IN THE LETTER OF TRANSMITTAL MAY
BE SUBJECT TO UNITED STATES FEDERAL INCOME TAX BACKUP WITHHOLDING EQUAL TO 31%
OF THE GROSS PROCEEDS PAYABLE TO SUCH SHAREHOLDER OR OTHER PAYEE PURSUANT TO
THE OFFER. SEE SECTIONS 3 AND 14.
 
  THE BOARD OF DIRECTORS HAS UNANIMOUSLY APPROVED THE OFFER. NEITHER THE
COMPANY NOR ITS BOARD OF DIRECTORS, HOWEVER, MAKES ANY RECOMMENDATION TO ANY
SHAREHOLDER AS TO WHETHER TO TENDER ALL OR ANY SHARES. EACH SHAREHOLDER MUST
MAKE HIS OR HER OWN DECISION AS TO WHETHER TO TENDER SHARES AND, IF SO, HOW
MANY SHARES TO TENDER AND AT WHAT PRICE. EMPLOYEES, OFFICERS AND DIRECTORS OF
THE COMPANY MAY PARTICIPATE IN THE OFFER ON THE SAME BASIS AS THE COMPANY'S
OTHER SHAREHOLDERS.
 
  As of January 15, 1998 the Company had issued and outstanding 14,143,655
Shares, and had reserved 542,000 Shares for issuance upon exercise of
currently exercisable stock options under the Company's Incentive
 
                                       1
<PAGE>
 
Stock Plan. The 1,500,000 Shares that the Company is offering to purchase
pursuant to the Offer represent approximately 11% of the Shares then
outstanding.
 
  The Shares are traded on the Nasdaq National Market. The Shares trade under
the symbol "STBA." On January 15, 1998 the closing price of the Shares on the
Nasdaq National Market was $43.75 per Share. See Section 7. SHAREHOLDERS ARE
URGED TO OBTAIN CURRENT MARKET QUOTATIONS FOR THE SHARES.
 
  Questions and requests for assistance may be directed to the Information
Agent:
 
                    Corporate Investor Communications, Inc.
                               111 Commerce Road
                              Carlstadt, NJ 07072
                     Banks and Brokers call (800) 346-7885
                   All others call Toll Free (800) 201-9605
 
                        1. NUMBER OF SHARES; PRORATION
 
  Upon the terms and subject to the conditions described herein and in the
Letter of Transmittal, the Company will purchase up to 1,500,000 Shares that
are validly tendered on or prior to the Expiration Date (as defined below)
(and not properly withdrawn in accordance with Section 4) at a price
(determined in the manner set forth below) not less than $46.00 nor in excess
of $49.00 per Share. The later of 5:00 p.m., Eastern time, on February 18,
1998, or the latest time and date to which the Offer is extended pursuant to
Section 15, is referred to herein as the "Expiration Date." If the Offer is
oversubscribed as described below, only Shares tendered at or below the
Purchase Price on or prior to the Expiration Date will be eligible for
proration. The proration period also expires on the Expiration Date.
 
  The Company will determine the Purchase Price taking into consideration the
number of Shares so tendered and the prices specified by tendering
shareholders. The Company will select the Purchase Price that will allow it to
purchase 1,500,000 Shares (or such lesser number of Shares as is validly
tendered and not withdrawn at prices not less than $46.00 nor in excess of
$49.00 per Share) pursuant to the Offer. Subject to Section 15, the Company
reserves the right to purchase more than 1,500,000 Shares pursuant to the
Offer, but does not currently plan to do so. THE OFFER IS NOT CONDITIONED ON
ANY MINIMUM NUMBER OF SHARES BEING TENDERED. THE OFFER IS, HOWEVER, SUBJECT TO
CERTAIN OTHER CONDITIONS. SEE SECTION 6.
 
  In accordance with Instruction 5 of the Letter of Transmittal, each
shareholder who wishes to tender Shares must specify the price (not less than
$46.00 nor in excess of $49.00 per Share) at which such shareholder is willing
to have the Company purchase such Shares. As promptly as practicable following
the Expiration Date, the Company will determine the Purchase Price (not less
than $46.00 nor in excess of $49.00 per Share) that it will pay for Shares
validly tendered and not withdrawn pursuant to the Offer, taking into account
the number of Shares so tendered and the prices specified by tendering
shareholders. All Shares purchased pursuant to the Offer will be purchased at
the Purchase Price. All Shares not purchased pursuant to the Offer, including
Shares tendered at prices greater than the Purchase Price and Shares not
purchased because of proration, will be returned to the tendering shareholders
at the Company's expense as promptly as practicable following the Expiration
Date.
 
  Upon the terms and subject to the conditions of the Offer, if 1,500,000 or
fewer Shares have been validly tendered at or below the Purchase Price and not
withdrawn on or prior to the Expiration Date, the Company will purchase all
such Shares. Upon the terms and subject to the conditions of the Offer, if
more than 1,500,000 Shares have been validly tendered at or below the Purchase
Price and not withdrawn on or prior to the Expiration Date, the Company will
purchase Shares in the following order of priority:
 
    (a)first, all Shares validly tendered at or below the Purchase Price and
  not withdrawn on or prior to the Expiration Date by or on behalf of any
  shareholder who owned beneficially, as of the close of business
 
                                       2
<PAGE>
 
  on January 15, 1998 and continues to own beneficially as of the Expiration
  Date, an aggregate of fewer than 100 Shares and who validly tenders all of
  such Shares (partial tenders will not qualify for this preference) and
  completes the box captioned "Odd Lots" on the Letter of Transmittal; and
 
    (b)then, after purchase of all of the foregoing Shares, all other Shares
  validly tendered at or below the Purchase Price and not withdrawn on or
  prior to the Expiration Date on a pro rata basis, if necessary (with
  appropriate adjustments to avoid purchases of fractional Shares).
 
  If proration of tendered Shares is required, (i) because of the difficulty
in determining the number of Shares validly tendered and (ii) as a result of
the "odd lot" procedure described in Section 2, the Company does not expect
that it would be able to announce the final proration factor or to commence
payment for any Shares purchased pursuant to the Offer until approximately
seven (7) trading days after the Expiration Date. Preliminary results of
proration will be announced by press release as promptly as practicable after
the Expiration Date. Holders of Shares also may obtain such preliminary
information from the Dealer Manager or the Information Agent.
 
  As described under "Section 14--Certain Federal Income Tax Consequences,"
the number of Shares that the Company will purchase from a shareholder may
affect the federal income tax consequences to the shareholder of such purchase
and therefore may be relevant to a shareholder's decision whether to tender
Shares.
 
  The Company expressly reserves the right, in its sole discretion, at any
time or from time to time, to extend the period of time during which the Offer
is open by giving oral or written notice of such extension to the Depositary
and making a public announcement thereof. See Section 15. There can be no
assurance, however, that the Company will exercise its right to extend the
Offer.
 
  For purposes of the Offer, a "business day" means any day other than a
Saturday, Sunday or federal holiday and consists of the time period from 12:01
a.m. through 12:00 midnight, Eastern time.
 
  Copies of this Offer to Purchase and the related Letter of Transmittal are
being mailed to record holders of Shares and will be furnished to brokers,
banks and similar persons whose names, or the names of whose nominees, appear
on the Company's shareholder list or, if applicable, who are listed as
participants in a clearing agency's security position listing for subsequent
transmittal to beneficial owners of Shares.
 
                2. TENDERS BY HOLDERS OF FEWER THAN 100 SHARES
 
  All Shares validly tendered at or below the Purchase Price and not withdrawn
on or prior to the Expiration Date by or on behalf of any shareholder who
owned beneficially, as of the close of business on January 15, 1998, and
continues to own beneficially as of the Expiration Date, an aggregate of fewer
than 100 Shares, will be accepted for purchase before proration, if any, of
other tendered Shares. Partial tenders will not qualify for this preference,
and it is not available to beneficial holders of 100 or more Shares, even if
such holders have separate stock certificates for fewer than 100 Shares. By
accepting the Offer, a shareholder owning beneficially fewer than 100 Shares
will avoid the payment of brokerage commissions and the applicable odd lot
discount payable in a sale of such Shares in a transaction effected on a
securities exchange.
 
  As of January 15, 1998, there were approximately 3,005 holders of record of
Shares. Approximately 579 of these holders of record held individually fewer
than 100 Shares and held in the aggregate approximately 19,595 Shares. Because
of the large number of Shares held in the names of brokers and nominees, the
Company is unable to determine the exact number of beneficial owners of fewer
than 100 Shares or the aggregate number of Shares they own. Any shareholder
wishing to tender all of his or her Shares pursuant to this Section should
complete the box captioned "Odd Lots" on the Letter of Transmittal.
 
  The special Odd Lot purchase rules described above do not apply to any
Shares held in the Thrift Plan for Employees of S&T Bancorp ("Thrift Plan").
 
 
                                       3
<PAGE>
 
                       3. PROCEDURE FOR TENDERING SHARES
 
  To tender Shares validly pursuant to the Offer, a properly completed and
duly executed Letter of Transmittal or facsimile thereof, together with any
required signature guarantees and any other documents required by the Letter
of Transmittal, must be received by the Depositary at its address set forth on
the back cover of this Offer to Purchase and either (i) certificates for the
Shares to be tendered must be received by the Depositary at such address, (ii)
such Shares must be delivered pursuant to the procedures for book-entry
transfer described below (and a confirmation of such delivery received by the
Depositary), or (iii) the tendering shareholder must comply with the
guaranteed delivery procedure described below, in each case on or prior to the
Expiration Date.
 
  IN ACCORDANCE WITH INSTRUCTION 5 OF THE LETTER OF TRANSMITTAL, IN ORDER TO
TENDER SHARES PURSUANT TO THE OFFER, A SHAREHOLDER MUST INDICATE IN THE
SECTION CAPTIONED "PRICE (IN DOLLARS) PER SHARE AT WHICH SHARES ARE BEING
TENDERED" ON THE LETTER OF TRANSMITTAL THE PRICE (IN MULTIPLES OF $0.50) AT
WHICH SUCH SHARES ARE BEING TENDERED.
 
  Shareholders wishing to tender Shares at more than one price must complete
separate Letters of Transmittal for each price at which such Shares are being
tendered. The same Shares cannot be tendered at more than one price. FOR A
TENDER OF SHARES TO BE VALID, A PRICE BOX, BUT ONLY ONE PRICE BOX, ON EACH
LETTER OF TRANSMITTAL MUST BE CHECKED.
 
  In addition, Odd Lot Holders who tender all such Shares must complete the
box captioned "Odd Lots" on the Letter of Transmittal and, if applicable, on
the Notice of Guaranteed Delivery, in order to qualify for the preferential
treatment available to Odd Lot Holders as described in Section 2.
 
  The Depositary will establish an account with respect to the Shares at the
Depositary Transfer Company ("DTC") (hereinafter referred to as the "Book-
Entry Transfer Facility") for purposes of the Offer within two business days
after the date of this Offer to Purchase, and any financial institution that
is a participant in the system of any Book-Entry Transfer Facility may make
delivery of Shares by causing such Book-Entry Transfer Facility to transfer
such Shares into the Depositary's account in accordance with the procedures of
such Book-Entry Transfer Facility. Although delivery of Shares may be effected
through book-entry transfer, a properly completed and duly executed Letter of
Transmittal or a manually signed copy thereof, or an Agent's Message (as
defined below), together with any required signature guarantees and any other
required documents, must, in any case, be transmitted to and received by the
Depositary at its address set forth on the back cover of this Offer to
Purchase on or prior to the Expiration Date. Delivery of required documents to
one of the Book-Entry Transfer Facilities in accordance with its procedures
does not constitute delivery to the Depositary and will not constitute a valid
tender.
 
  The term "Agent's Message" means a message transmitted by a Book-Entry
Transfer Facility to, and received by, the Depositary and forming a part of a
Book-Entry confirmation, which states that such Book-Entry Transfer Facility
has received an express acknowledgement from the participant in such Book-
Entry Transfer Facility tendering the Shares, that such participant has
received and agrees to be bound by the terms of the Letter of Transmittal and
that the Company may enforce such agreement against the participant.
 
  Except as set forth below, all signatures on a Letter of Transmittal must be
guaranteed by a firm that is a member of a registered national securities
exchange or the National Association of Securities Dealers, Inc., or by a
commercial bank, a trust company, a savings bank, a savings and loan
association or a credit union which has membership in an approved Signature
Guarantee Medallion Program (each of the foregoing being referred to as an
"Eligible Institution"). Signatures on a Letter of Transmittal need not be
guaranteed if (a) the Letter of Transmittal is signed by the registered holder
of the Shares (which term, for the purposes of this Section, includes any
participant in any Book-Entry Transfer Facility whose name appears on a
security position listing as the holder of the Shares) tendered therewith and
such holder has not completed the box entitled "Special Payment
 
                                       4
<PAGE>
 
Instructions" or the box entitled "Special Delivery Instructions" on the
Letter of Transmittal or (b) such Shares are tendered for the account of an
Eligible Institution. See Instructions 1 and 6 of the Letter of Transmittal.
 
  If a shareholder desires to tender Shares pursuant to the Offer and such
shareholder's certificates are not immediately available (or the procedures
for book-entry transfer cannot be completed on a timely basis) or time will
not permit all required documents to reach the Depositary by the Expiration
Date, such Shares may nevertheless be tendered provided that all of the
following conditions are satisfied:
 
    (a)such tender is made by or through an Eligible Institution;
 
    (b)the Depositary receives (by hand, mail, telegram or facsimile
  transmission), on or prior to the Expiration Date, a properly completed and
  duly executed Notice of Guaranteed Delivery substantially in the form the
  Company has provided with this Offer to Purchase (indicating the price at
  which the Shares are being tendered) and includes a guarantee by an
  Eligible Institution in the form set forth in such Notice; and
 
    (c)the certificates for all tendered Shares in proper form for transfer
  (or confirmation of book-entry transfer of such Shares into the
  Depositary's account at one of the Book-Entry Transfer Facilities),
  together with a properly completed and duly executed Letter of Transmittal
  (or facsimile thereof) and any other documents required by the Letter of
  Transmittal, are received by the Depositary within three Nasdaq trading
  days after the date the Depositary receives such Notice of Guaranteed
  Delivery.
 
  THE METHOD OF DELIVERY OF SHARES AND ALL OTHER REQUIRED DOCUMENTS IS AT THE
OPTION AND RISK OF THE TENDERING SHAREHOLDER. IF DELIVERY IS BY MAIL,
REGISTERED MAIL WITH RETURN RECEIPT REQUESTED, PROPERLY INSURED, IS
RECOMMENDED. IN ALL CASES SUFFICIENT TIME SHOULD BE ALLOWED TO ASSURE TIMELY
DELIVERY.
 
  TO PREVENT UNITED STATES FEDERAL INCOME TAX BACKUP WITHHOLDING EQUAL TO 31%
OF THE GROSS PAYMENTS MADE PURSUANT TO THE OFFER, EACH TENDERING SHAREHOLDER
MUST PROVIDE THE DEPOSITARY WITH SUCH SHAREHOLDER'S CORRECT TAXPAYER
IDENTIFICATION NUMBER AND CERTAIN OTHER INFORMATION BY PROPERLY COMPLETING THE
SUBSTITUTE FORM W-9 INCLUDED IN THE LETTER OF TRANSMITTAL. FOREIGN
SHAREHOLDERS (AS DEFINED IN SECTION 14) MUST SUBMIT A PROPERLY COMPLETED FORM
W-8 (WHICH MAY BE OBTAINED FROM THE DEPOSITARY) IN ORDER TO PREVENT BACKUP
WITHHOLDING. IN GENERAL, BACKUP WITHHOLDING DOES NOT APPLY TO CORPORATIONS OR
TO FOREIGN SHAREHOLDERS SUBJECT TO 30% (OR LOWER TREATY RATE) WITHHOLDING ON
GROSS PAYMENTS RECEIVED PURSUANT TO THE OFFER (AS DISCUSSED IN SECTION 14).
For a discussion of certain federal income tax consequences to tendering
shareholders, see Section 14. EACH SHAREHOLDER IS URGED TO CONSULT WITH HIS OR
HER OWN TAX ADVISOR REGARDING HIS, HER OR ITS QUALIFICATION FOR EXEMPTION FROM
BACKUP WITHHOLDING AND THE PROCEDURE FOR OBTAINING ANY APPLICABLE EXEMPTION.
 
  The Thrift Plan includes a Company stock investment fund and an employee
stock ownership plan ("ESOP") account. As of January 15, 1998, a total of
244,461 shares were held in the Company stock investment fund, all of which
were allocated to participant accounts, and 277,254 shares were held by the
ESOP, of which 264,254 were allocated to participant accounts. Shares
allocated to participants' accounts will, subject to the limitations of the
Employee Retirement Income Security Act of 1974, as amended, and applicable
regulations thereunder ("ERISA"), be tendered by the trustees of the Thrift
Plan according to the instructions of participants. The trustees will make
available to the participants whose accounts hold allocated Shares all
documents furnished to shareholders of the Company in connection with the
Offer generally and will provide additional information in separate letters
with respect to the operations of the Offer to the participants. Each such
participant also will receive a form upon which the participant may instruct
the trustees regarding the Offer. Each
 
                                       5
<PAGE>
 
participant may direct that all or some of the Shares allocated to the
participants' accounts be tendered. Participants also will be afforded
withdrawal rights. See "Section 4--Withdrawal Rights."
 
  Under ERISA the Company will be prohibited from purchasing any Shares from
the Thrift Plan (including Shares allocated to the accounts of participants)
if the Purchase Price is less than the prevailing market price of the Shares
on the date the Shares are accepted for payment pursuant to the Offer. If
Shares tendered from the Thrift Plan would have been accepted pursuant to the
terms of the Offer except for this prohibition, such Shares shall
automatically be deemed to be withdrawn.
 
  Shareholders who participate in the Company's Dividend Reinvestment Plan who
want to tender Shares under that plan should mark the appropriate box on the
Letter of Transmittal and follow the relevant instructions therein.
 
  It is a violation of Rule 14e-4 promulgated under the Securities Exchange
Act of 1934, as amended (the "Exchange Act"), for a person to tender Shares
for his or her own account unless the person so tendering (i) has a net long
position equal to or greater than the amount of (x) Shares tendered or (y)
other securities immediately convertible into, exercisable or exchangeable for
the amount of Shares tendered and will acquire such Shares for tender by
conversion, exercise or exchange of such other securities and (ii) will cause
such Shares to be delivered in accordance with the terms of the Offer. Rule
14e-4 provides a similar restriction applicable to the tender on behalf of
another person. The tender of Shares pursuant to any one of the procedures
described above will constitute the tendering shareholder's representation and
warranty that (i) such shareholder has a net long position in the Shares being
tendered within the meaning of Rule 14e-4 promulgated under the Exchange Act,
and (ii) the tender of such Shares complies with Rule 14e-4. The Company's
acceptance for payment of Shares tendered pursuant to the Offer will
constitute a binding agreement between the tendering shareholder and the
Company upon the terms and subject to the conditions of the Offer.
 
  All questions as to the Purchase Price, the form of documents, the number of
Shares to be accepted and the validity, eligibility (including time of
receipt) and acceptance for payment of any tender of Shares will be determined
by the Company, in its sole discretion, which determination shall be final and
binding on all parties. The Company reserves the absolute right to reject any
or all tenders of Shares that it determines are not in proper form or the
acceptance for payment of or payment for Shares that may, in the opinion of
the Company's counsel, be unlawful. The Company also reserves the absolute
right to waive any defect or irregularity in any tender of any particular
Shares. None of the Company, the Dealer Manager, the Depositary, the
Information Agent or any other person is or will be under any duty to give
notice of any defect or irregularity in tenders, nor shall any of them incur
any liability for failure to give any such notice.
 
  CERTIFICATES FOR SHARES, TOGETHER WITH A PROPERLY COMPLETED LETTER OF
TRANSMITTAL (OR, IN THE CASE OF A BOOK-ENTRY TRANSFER, AN AGENT'S MESSAGE) AND
ANY OTHER DOCUMENTS REQUIRED BY THE LETTER OF TRANSMITTAL, MUST BE DELIVERED
TO THE DEPOSITARY AND NOT TO THE COMPANY. ANY SUCH DOCUMENTS DELIVERED TO THE
COMPANY WILL NOT BE FORWARDED TO THE DEPOSITARY AND THEREFORE WILL NOT BE
PROPERLY TENDERED.
 
                             4. WITHDRAWAL RIGHTS
 
  Tenders of Shares made pursuant to the Offer may be withdrawn at any time
prior to the Expiration Date. Thereafter, such tenders are irrevocable, except
that they may be withdrawn after 12:00 midnight, Eastern time, March 17, 1998
unless theretofore accepted for payment by the Company as provided in this
Offer to Purchase. If the Company extends the period of time during which the
Offer is open, is delayed in purchasing Shares or is unable to purchase Shares
pursuant to the Offer for any reason, then, without prejudice to the Company's
rights under the Offer, the Depositary may, on behalf of the Company, retain
all Shares tendered, and such Shares may not be withdrawn except as otherwise
provided in this Section 4, subject to Rule 13e-4(f)(5) under the Exchange
Act, which provides that the issuer making the tender offer shall either pay
the consideration offered, or return the tendered securities promptly after
the termination or withdrawal of the tender offer.
 
                                       6
<PAGE>
 
  Tenders of Shares made pursuant to the Offer may not be withdrawn after the
Expiration Date, except that they may be withdrawn after 12:00 midnight,
Eastern time, March 17, 1998, unless accepted for payment by the Company as
provided in this Offer to Purchase. For a withdrawal to be effective prior to
that time, a shareholder of Shares held in physical form must provide a
written, telegraphic or facsimile transmission notice of withdrawal to the
Depositary at its address set forth on the back cover page of this Offer to
Purchase before the Expiration Date, which notice must contain: (A) the name
of the person who tendered the Shares; (B) a description of the Shares to be
withdrawn (including the number of Shares being withdrawn); (C) the
certificate numbers shown on the particular certificates evidencing such
Shares; (D) the signature of such shareholder executed in the same manner as
the original signature on the Letter of Transmittal (including any signature
guarantee (if such original signature was guaranteed)); and (E) if such Shares
are held by a new beneficial owner, evidence satisfactory to the Company that
the person withdrawing the tender has succeeded to the beneficial ownership of
the Shares. A purported notice of withdrawal which lacks any of the required
information will not be an effective withdrawal of a tender previously made.
 
  A shareholder of Shares held with any of the Book-Entry Transfer Facilities
must call such shareholder's broker and instruct such broker to withdraw such
tender of Shares and instruct such broker to provide a written, telegraphic or
facsimile transmission notice of withdrawal to the Depositary on or before the
Expiration Date. A purported notice of withdrawal which lacks any of the
applicable required information noted above will not be an effective
withdrawal of a tender previously made.
 
  Any permitted withdrawals of tenders of Shares may not be rescinded, and any
Shares so withdrawn will thereafter be deemed not validly tendered for
purposes of the Offer; provided, however, that withdrawn Shares may be re-
tendered by following the procedures for tendering prior to the Expiration
Date.
 
  All questions as to the form and validity (including time of receipt) of any
notice of withdrawal will be determined by the Company, in its sole
discretion, which determination shall be final and binding on all parties.
None of the Company, the Dealer Manager, the Depositary, the Information Agent
or any other person is or will be under any duty to give notification of any
defect or irregularity in any notice of withdrawal or incur any liability for
failure to give any such notification.
 
       5. ACCEPTANCE FOR PAYMENT OF SHARES AND PAYMENT OF PURCHASE PRICE
 
  Upon the terms and subject to the conditions of the Offer and as promptly as
practicable after the Expiration Date, the Company will determine the Purchase
Price, taking into consideration the number of Shares tendered and the prices
specified by tendering shareholders, announce the Purchase Price, and (subject
to the proration provisions of the Offer) accept for payment and pay the
Purchase Price for Shares validly tendered and not withdrawn at or below the
Purchase Price. Thereafter, payment for all Shares validly tendered on or
prior to the Expiration Date and accepted for payment pursuant to the Offer
will be made by the Depositary by check as promptly as practicable. In all
cases, payment for Shares accepted for payment pursuant to the Offer will be
made only after timely receipt by the Depositary of certificates for such
Shares (or of a timely confirmation of a book-entry transfer of such Shares
into the Depositary's account at one of the Book-Entry Transfer Facilities), a
properly completed and duly executed Letter of Transmittal or a manually
signed copy thereof, with any required signature guarantees, or, in the case
of a book-entry delivery an Agent's Message, and any other required documents.
 
  For purposes of the Offer, the Company shall be deemed to have accepted for
payment (and thereby purchased), subject to proration, Shares that are validly
tendered and not withdrawn as, if and when it gives oral or written notice to
the Depositary of the Company's acceptance for payment of such Shares. In the
event of proration, the Company will determine the proration factor and pay
for those tendered Shares accepted for payment as soon as practicable after
the Expiration Date. However, the Company does not expect to be able to
announce the final results of any such proration until approximately seven (7)
trading days after the Expiration Date. The Company will pay for Shares that
it has purchased pursuant to the Offer by depositing the aggregate
 
                                       7
<PAGE>
 
Purchase Price therefor with the Depositary. The Depositary will act as agent
for tendering shareholders for the purpose of receiving payment from the
Company and transmitting payment to tendering shareholders. Under no
circumstances will interest be paid on amounts to be paid to tendering
shareholders, regardless of any delay in making such payment.
 
  Certificates for all Shares not purchased, including all Shares tendered at
prices greater than the Purchase Price and Shares not purchased because of
proration, will be returned (or, in the case of Shares tendered by book-entry
transfer, such Shares will be credited to an account maintained with the Book-
Entry Transfer Facility by the participant therein who so delivered such
Shares) as promptly as practicable following the Expiration Date without
expense to the tendering shareholder.
 
  Payment for Shares may be delayed in the event of difficulty in determining
the number of Shares properly tendered or if proration is required. See
Section 1. In addition, if certain events occur, the Company may not be
obligated to purchase Shares pursuant to the Offer. See Section 6.
 
  The Company will pay or cause to be paid any stock transfer taxes with
respect to the sale and transfer of any Shares to it or its order pursuant to
the Offer. If, however, payment of the Purchase Price is to be made to, or a
portion of the Shares delivered (whether in certificated form or by book
entry) but not tendered or not purchased are to be registered in the name of,
any person other than the registered holder, or if tendered Shares are
registered in the name of any person other than the person signing the Letter
of Transmittal (unless such person is signing in a representative or fiduciary
capacity), the amount of any stock transfer taxes (whether imposed on the
registered holder, such other person or otherwise) payable on account of the
transfer to such person will be deducted from the Purchase Price unless
satisfactory evidence of the payment of such taxes, or exemption therefrom, is
submitted. See Instruction 7 to the Letter of Transmittal.
 
  ANY TENDERING SHAREHOLDER OR OTHER PAYEE WHO FAILS TO COMPLETE FULLY AND
SIGN THE SUBSTITUTE FORM W-9 INCLUDED IN THE LETTER OF TRANSMITTAL (OR, IN THE
CASE OF A FOREIGN INDIVIDUAL, A FORM W-8) MAY BE SUBJECT TO REQUIRED FEDERAL
INCOME TAX WITHHOLDING OF 31% OF THE GROSS PROCEEDS PAID TO SUCH SHAREHOLDER
OR OTHER PAYEE PURSUANT TO THE OFFER. SEE SECTION 3.
 
                      6. CERTAIN CONDITIONS OF THE OFFER
 
  Notwithstanding any other provision of the Offer, the Company will not be
required to accept for payment or pay for any Shares tendered, and may
terminate or amend and may postpone (subject to the requirements of the
Exchange Act for prompt payment for or return of Shares tendered) the
acceptance for payment of Shares tendered, if at any time after January 20,
1998 and at or before acceptance for payment of any Shares any of the
following shall have occurred:
 
    (a)there shall have been threatened, instituted or pending any action or
  proceeding by any government or governmental, regulatory or administrative
  agency or authority or tribunal or any other person, domestic or foreign,
  or before any court, authority, agency or tribunal that (i) challenges the
  acquisition of Shares pursuant to the Offer or otherwise in any manner
  relates to or affects the Offer or (ii) in the sole judgment of the
  Company, could materially and adversely affect the business, condition
  (financial or other), income, operations or prospects of the Company and
  its subsidiaries, taken as a whole, or otherwise materially impair in any
  way the contemplated future conduct of the business of the Company or any
  of its subsidiaries or materially impair the Offer's contemplated benefits
  to the Company;
 
    (b)there shall have been any action threatened, pending or taken, or
  approval withheld, or any statute, rule, regulation, judgment, order or
  injunction threatened, proposed, sought, promulgated, enacted, entered,
  amended, enforced or deemed to be applicable to the Offer or the Company or
  any of its subsidiaries, by any legislative body, court, authority, agency
  or tribunal which, in the Company's sole judgment, would or might directly
  or indirectly (i) make the acceptance for payment of, or payment for, some
  or all of the Shares
 
                                       8
<PAGE>
 
  illegal or otherwise restrict or prohibit consummation of the Offer, (ii)
  delay or restrict the ability of the Company, or render the Company unable,
  to accept for payment or pay for some or all of the Shares, (iii)
  materially impair the contemplated benefits of the Offer to the Company or
  (iv) materially affect the business, condition (financial or other),
  income, operations or prospects of the Company and its subsidiaries, taken
  as a whole, or otherwise materially impair in any way the contemplated
  future conduct of the business of the Company or any of its subsidiaries;
 
    (c)there shall have occurred (i) any general suspension of trading in, or
  limitation on prices for, securities on any national securities exchange or
  in the over-the-counter market, (ii) any significant decline in the market
  price of the Shares or in the general level of market prices of equity
  securities in the United States or abroad, (iii) any change in the general
  political, market, economic or financial condition in the United States or
  abroad that could have a material adverse effect on the Company's business,
  condition (financial or otherwise), income, operations, prospects or
  ability to obtain financing generally or the trading in the Shares, (iv)
  the declaration of a banking moratorium or any suspension of payments in
  respect of banks in the United States or any limitation on, or any event
  which, in the Company's sole judgment, might affect the extension of credit
  by lending institutions in the United States, (v) the commencement of a
  war, armed hostilities or other international or national calamity directly
  or indirectly involving the United States or (vi) in the case of any of the
  foregoing existing at the time of the commencement of the Offer, in the
  Company's sole judgment, a material acceleration or worsening thereof;
 
    (d)a tender or exchange offer with respect to some or all of the Shares
  (other than the Offer), or a merger, acquisition or other business
  combination proposal for the Company, shall have been proposed, announced
  or made by another person or group (within the meaning of Section 13(d)(3)
  of the Exchange Act);
 
    (e)there shall have occurred any event or events that has resulted, or
  may in the sole judgment of the Company result, directly or indirectly, in
  an actual or threatened change in the business, condition (financial or
  other), income, operations, stock ownership or prospects of the Company and
  its subsidiaries;
 
and, in the sole judgment of the Company, such event or events make it
undesirable or inadvisable to proceed with the Offer or with such acceptance
for payment.
 
  The foregoing conditions are for the sole benefit of the Company and may be
asserted by the Company regardless of the circumstances (including any action
or inaction by the Company) giving rise to any such condition, and any such
condition may be waived by the Company, in whole or in part, at any time and
from time to time in its sole discretion. The failure by the Company at any
time to exercise any of the foregoing rights shall not be deemed a waiver of
any such right and each such right shall be deemed an ongoing right which may
be asserted at any time and from time to time. Any determination by the
Company concerning the events described above will be final and binding on all
parties.
 
  Acceptance of Shares validly tendered in the Offer is subject to the
condition that, as of the Expiration Date, and after giving pro forma effect
to the acceptance of Shares validly tendered, the Company would continue to
have at least 300 shareholders of record and the Shares would remain listed
for quotation on the Nasdaq National Market. This condition may not be waived.
 
  The Exchange Act requires that all conditions to the Offer must be satisfied
or waived before the Expiration Date.
 
 
                                       9
<PAGE>
 
                      7. PRICE RANGE OF SHARES; DIVIDENDS
 
  The following table sets forth the high and low sales prices, and dividends
declared, for the shares as reported on the Nasdaq National Market for the
periods indicated.
 
<TABLE>
<CAPTION>
                                                                       DIVIDENDS
                                                        HIGH     LOW   DECLARED
                                                       ------- ------- ---------
<S>                                                    <C>     <C>     <C>
Fiscal 1996
1st Quarter........................................... $30 3/4 $28       $0.21
2nd Quarter...........................................  31 1/4  29 1/2    0.24
3rd Quarter...........................................  31 3/4  29 5/8    0.24
4th Quarter...........................................  31 3/4  30        0.25
Fiscal 1997
1st Quarter........................................... $36 3/4 $29 3/4   $0.25
2nd Quarter...........................................  36 7/8  29 1/2    0.28
3rd Quarter...........................................  38 3/4  33 1/4    0.28
4th Quarter...........................................  44 1/2  38 1/2    0.30
</TABLE>
 
  On January 15, 1998, the closing price of the Shares on the Nasdaq National
Market was $43.75 per Share. SHAREHOLDERS ARE URGED TO OBTAIN CURRENT MARKET
QUOTATIONS FOR THE SHARES.
 
             8. PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER
 
  The Company believes that the purchase of Shares is an attractive use of a
portion of the Company's available capital on behalf of its shareholders and
is consistent with the Company's long-term goal of increasing shareholder
value. The Company believes it has adequate sources of capital to complete the
Share repurchase and pursue business opportunities.
 
  Over time, the Company's profitable operations have contributed to the
growth of a capital base that exceeds all applicable regulatory standards and
the amount of capital needed to support the Company's banking business. After
evaluating a variety of alternatives to utilize more effectively its capital
base and to attempt to maximize shareholder value, the Company's management
and its Board of Directors believe that the purchase of Shares pursuant to the
Offer is a positive action that is intended to accomplish the desired
objective of increasing shareholder value.
 
  Other actions previously employed, including an open market purchase of
Shares, have enhanced shareholder value, but capital remains at high levels.
The Offer is designed to restructure the Company's balance sheet in order to
increase return on equity by reducing the amount of equity outstanding. Based
upon the current market price of its Shares, the Company believes that the
purchase of Shares is an attractive use of its funds. Following the purchase
of the Shares, the Company believes funds provided by earnings, combined with
its other sources of liquidity, will be fully adequate to meet its funding
needs for the foreseeable future. Upon completion of the Offer, the Company
expects that the Company and its wholly owned subsidiary bank, S&T Bank (the
"Bank"), will continue to maintain the highest regulatory standard for
capital, which is designated as "well capitalized" under the prompt corrective
action scheme enacted by the Federal Deposit Insurance Corporation Improvement
Act of 1991.
 
  The Offer will enable shareholders who are considering the sale of all or a
portion of their Shares the opportunity to determine the price or prices (not
less than $46.00 nor in excess of $49.00 per Share) at which they are willing
to sell their Shares, and, if any such Shares are purchased pursuant to the
Offer, to sell those Shares for cash without the usual transaction costs
associated with open-market sales. The Offer may also give shareholders the
opportunity to sell Shares at prices greater than market prices prevailing
prior to the announcement of the Offer. See Section 7. In addition, qualifying
shareholders owning beneficially fewer than 100 Shares, whose Shares are
purchased pursuant to the Offer, not only will avoid the payment of brokerage
 
                                      10
<PAGE>
 
commissions but will also avoid any applicable odd lot discounts to the market
price typically charged by brokers for executing odd lot trades.
 
  Shareholders who do not tender their Shares pursuant to the Offer and
shareholders who otherwise retain an equity interest in the Company as a
result of a partial tender of Shares or a proration pursuant to Section 1 of
the Offer will continue to be owners of the Company with the attendant risks
and rewards associated with owning the equity securities of the Company. As
noted above, the Company, following completion of the Offer, will maintain the
highest regulatory capital ranking. Consequently, the Company believes that
shareholders will not be subject to materially greater risk as a result of the
reduction of the capital base.
 
  Shareholders who determine not to accept the Offer will realize a
proportionate increase in their relative equity interest in the Company and,
thus, in the Company's earnings and assets, subject to any risks resulting
from the Company's purchase of Shares and the Company's ability to issue
additional equity securities in the future. In addition, to the extent the
purchase of Shares pursuant to the Offer results in a reduction of the number
of shareholders of record, the Company's costs for services to shareholders
may be reduced. Finally, the Offer may affect the Company's ability to qualify
for pooling-of-interests accounting treatment for any merger transaction for
approximately the next two years.
 
  If fewer than 1,500,000 Shares are purchased pursuant to the Offer, the
Company may repurchase the remainder of such Shares on the open market, in
privately negotiated transactions or otherwise. In the future, the Company may
determine to purchase additional Shares on the open market, in privately
negotiated transactions, through one or more tender offers or otherwise. Any
such purchases may be on the same terms as, or on terms which are more or less
favorable to shareholders than, the terms of the Offer. However, Rule 13e-4
under the Exchange Act prohibits the Company and its affiliates from
purchasing any Shares, other than pursuant to the Offer, until at least ten
business days after the Expiration Date. Any future purchases of Shares by the
Company would depend on many factors, including the market price of the
Shares, the Company's business and financial position, and general economic
and market conditions.
 
  Shares the Company acquires pursuant to the Offer will be restored to the
status of authorized and unissued Shares, or placed in the Company's treasury,
and will be available for the Company to issue without further shareholder
action (except as required by applicable law or the rules of the Nasdaq
National Market or any other securities exchange on which the Shares are
listed) for purposes including, but not limited to, the acquisition of other
businesses, the raising of additional capital for use in the Company's
business and the satisfaction of obligations under existing or future employee
benefit plans. The Company has no current plans for reissuance of the Shares
repurchased pursuant to the Offer.
 
  NEITHER THE COMPANY NOR ITS BOARD OF DIRECTORS MAKES ANY RECOMMENDATION TO
ANY SHAREHOLDER AS TO WHETHER TO TENDER ALL OR ANY SHARES. EACH SHAREHOLDER
MUST MAKE HIS OR HER OWN DECISION WHETHER TO TENDER SHARES AND, IF SO, HOW
MANY SHARES TO TENDER AND AT WHAT PRICE. EMPLOYEES, OFFICERS AND DIRECTORS OF
THE COMPANY MAY PARTICIPATE IN THE OFFER ON THE SAME BASIS AS THE COMPANY'S
OTHER SHAREHOLDERS.
 
                 9. CERTAIN INFORMATION CONCERNING THE COMPANY
 
GENERAL
 
  The Company was incorporated on March 17, 1983 under the laws of the
Commonwealth of Pennsylvania as a bank holding company and has two wholly
owned subsidiaries, S&T Bank and S&T Investment Company, Inc. The Company is
registered as a bank holding company with the Board of Governors of the
Federal Reserve System under the Bank Holding Company Act, as amended. As of
September 30, 1997, the Company had $1.8 billion in total assets, $250 million
in total shareholders' equity and $1.3 billion in total deposits.
 
 
                                      11
<PAGE>
 
  S&T Bank is a full service bank with its main office at 800 Philadelphia
Street, Indiana, Pennsylvania, providing service to its customers through a
branch network of thirty-eight offices located in Armstrong, Allegheny,
Indiana, Jefferson, Clearfield and Westmoreland counties.
 
  S&T Bank's services include accepting time and demand deposit accounts,
making secured and unsecured commercial and consumer loans, providing letters
of credit, and offering discount brokerage services, personal financial
planning and credit card services. S&T Bank has a relatively stable deposit
base and no material amount of deposits is obtained from a single depositor or
group of depositors (including federal, state and local governments). S&T Bank
does not experience significant fluctuations in deposits. Deposits held by S&T
Bank are insured by the Federal Deposit Insurance Corporation to the full
extent provided by law.
 
                                      12
<PAGE>
 
              SUMMARY HISTORICAL CONSOLIDATED FINANCIAL DATA AND
            SUMMARY UNAUDITED PRO FORMA CONSOLIDATED FINANCIAL DATA
 
  The following summary historical consolidated financial data has been
derived from the unaudited supplemental consolidated financial statements of
the Company and, in the opinion of management, include all adjustments
(consisting of normal recurring adjustments) necessary for a fair presentation
of financial position and results of operations for such periods. All prior
period amounts have been restated to reflect the pooling of interests
transaction with Peoples Bank of Unity which closed on May 2, 1997. The data
should be read in conjunction with the consolidated financial statements and
notes thereto included in the Company's Quarterly Report on Form 10-Q for the
quarter ended September 30, 1997 and the Company's Annual Report on Form 10-K
for the year ended December 31, 1996. Copies of these reports may be obtained
as described in Section 17 of this Offer.
 
  On January 20, 1998, the company announced earnings results for the twelve
months ended December 31, 1997 of $33,414,000 or $2.36 per share as compared
to $28,241,000 or $2.00 per share for the year ending December 31, 1996. Net
income for the quarter ending December 31, 1997 was $8,711,000 or $0.62 per
share as compared to $6,938,000 or $0.49 per share for the same period 1996. A
copy of the Company's press release announcing 1997 earnings accompanies this
Offer to Purchase.
 
  The following summary unaudited pro forma consolidated financial data has
been derived from the historical consolidated financial statements of the
Company adjusted for certain costs and expenses to be incurred as a result of
the purchase of Shares pursuant to the Offer. The summary unaudited pro forma
consolidated financial data should be read in conjunction with the summary
historical consolidated financial data included herein. The pro forma income
statement data and balance sheet data are not necessarily indicative of the
financial position or results of operations that would have been obtained had
the Offer been completed as of the dates indicated.
 
 
 
                                      13
<PAGE>
 
                               S&T BANCORP, INC.
                     SUMMARY HISTORICAL BALANCE SHEET DATA
 
<TABLE>
<CAPTION>
                                                               NINE MONTHS
                                  YEAR ENDED DECEMBER 31,  ENDED SEPTEMBER 30,
                                  ----------------------- ---------------------
                                        1996        1995     1997       1996
                                  ----------- ----------- ---------- ----------
                                      (IN THOUSANDS, EXCEPT PER SHARE DATA)
<S>                               <C>         <C>         <C>        <C>
ASSETS
  Securities..................... $   500,061     492,236    490,565 $  494,026
  Net loans......................   1,181,407   1,086,317  1,238,663  1,141,572
  Other assets...................     105,577     111,175    102,924    107,353
                                  ----------- ----------- ---------- ----------
    Total Assets................. $ 1,787,045 $ 1,689,728 $1,832,152 $1,742,951
                                  =========== =========== ========== ==========
LIABILITIES
  Total deposits................. $ 1,270,367 $ 1,216,547  1,289,891  1,247,002
  Securities sold under
   repurchase agreements.........     114,205     122,794    121,425    144,915
  Other liabilities..............     176,355     136,333    170,704    133,202
                                  ----------- ----------- ---------- ----------
    Total Liabilities             $ 1,560,927 $ 1,475,674 $1,582,020 $1,525,119
SHAREHOLDERS' EQUITY.............     226,118     214,054    250,132    217,832
                                  ----------- ----------- ---------- ----------
    Total Liabilities and
     Shareholders' Equity........ $ 1,787,045 $ 1,689,728 $1,832,152 $1,742,951
                                  =========== =========== ========== ==========
</TABLE>
 
 
                                       14
<PAGE>
 
                               S&T BANCORP, INC.
            UNAUDITED PRO FORMA CONDENSED CONSOLIDATED BALANCE SHEET
                               DECEMBER 31, 1996
                     (IN THOUSANDS, EXCEPT PER SHARE DATA)
 
<TABLE>
<CAPTION>
                                                         SHARES PURCHASED AT
                                                        ----------------------
                                                           $46         $49
                                                        PER SHARE   PER SHARE
                                                        ----------  ----------
<S>                                                     <C>         <C>
ASSETS
  Cash and due from banks.............................. $   40,710  $   40,710
  Interest-earning deposits with banks.................        109         109
  Federal funds sold...................................      6,465       6,465
  Securities:
    Available for sale (3).............................    439,801     439,801
    Held to maturity...................................     50,260      50,260
                                                        ----------  ----------
  Total securities.....................................    490,061     490,061
  Net loans............................................  1,181,407   1,181,407
  Premises and equipment...............................     20,038      20,038
  Other assets.........................................     38,255      38,255
                                                        ----------  ----------
      Total Assets..................................... $1,777,045  $1,777,045
                                                        ==========  ==========
LIABILITIES
  Total deposits....................................... $1,270,367  $1,270,367
  Securities sold under repurchase agreements..........    114,205     114,205
  Federal funds purchased..............................        775         775
  Other borrowed funds.................................        230         230
  Long term borrowings (3).............................    195,618     200,118
  Other liabilities....................................     38,732      38,732
                                                        ----------  ----------
      Total Liabilities................................ $1,619,927  $1,624,427
SHAREHOLDERS' EQUITY
  Preferred stock, without par value, 10,000,000 shares
   authorized
   and none outstanding................................        --          --
  Common stock $2.50 par value, 25,000,000 shares
   authorized
   and 14,857,019 issued...............................     37,142      37,142
  Additional paid in capital...........................     19,044      19,044
  Retained earnings....................................    157,982     157,982
  Net unrealized holding gains on securities available
   for sale............................................     25,197      25,197
  Treasury stock (1)(2)(4).............................    (82,017)    (86,517)
  Deferred compensation................................       (230)       (230)
                                                        ----------  ----------
      Total Shareholders' Equity.......................    157,118     152,618
                                                        ----------  ----------
      Total Liabilities and Shareholders' Equity....... $1,777,045  $1,777,045
                                                        ==========  ==========
</TABLE>
 
                                       15
<PAGE>
 
                               S&T BANCORP, INC.
            UNAUDITED PRO FORMA CONDENSED CONSOLIDATED BALANCE SHEET
                               SEPTEMBER 30, 1997
                     (IN THOUSANDS, EXCEPT PER SHARE DATA)
 
<TABLE>
<CAPTION>
                                                         SHARES PURCHASED AT
                                                        ----------------------
                                                           $46         $49
                                                        PER SHARE   PER SHARE
                                                        ----------  ----------
<S>                                                     <C>         <C>
ASSETS
  Cash and due from banks.............................. $   43,400  $   43,400
  Interest-earning deposits with banks.................        104         104
  Federal funds sold...................................          0           0
  Securities:
    Available for sale (3).............................    431,266     431,266
    Held to maturity...................................     49,299      49,299
                                                        ----------  ----------
  Total securities.....................................    480,565     480,565
  Net loans............................................  1,238,663   1,238,663
  Premises and equipment...............................     19,235      19,235
  Other assets.........................................     40,185      40,185
                                                        ----------  ----------
      Total Assets..................................... $1,822,152  $1,822,152
                                                        ==========  ==========
LIABILITIES
  Total deposits....................................... $1,289,891  $1,289,891
  Securities sold under repurchase agreements..........    121,425     121,425
  Federal funds purchased..............................        935         935
  Other borrowed funds.................................        230         230
  Long term borrowings (3).............................    182,118     186,618
  Other liabilities....................................     46,421      46,421
                                                        ----------  ----------
      Total Liabilities................................ $1,641,020  $1,645,520
SHAREHOLDERS' EQUITY
  Preferred stock, without par value, 10,000,000 shares
   authorized
   and none outstanding................................         --          --
  Common stock $2.50 par value, 25,000,000 shares
   authorized
   and 14,857,019 issued...............................     37,142      37,142
  Additional paid in capital...........................     19,333      19,333
  Retained earnings....................................    171,239     171,239
  Net unrealized holding gains on securities available
   for sale............................................     35,212      35,212
  Treasury stock (1)(2)(4).............................    (81,564)    (86,064)
  Deferred compensation................................       (230)       (230)
                                                        ----------  ----------
      Total Shareholders' Equity.......................    181,132     176,632
                                                        ----------  ----------
      Total Liabilities And Shareholders' Equity....... $1,822,152  $1,822,152
                                                        ==========  ==========
</TABLE>
 
                                       16
<PAGE>
 
                               S&T BANCORP, INC.
                    SUMMARY HISTORICAL INCOME STATEMENT DATA
 
<TABLE>
<CAPTION>
                                               YEAR ENDED     NINE MONTHS ENDED
                                              DECEMBER 31,      SEPTEMBER 30,
                                            ----------------- ------------------
                                              1996     1995       1997     1996
                                            -------- -------- --------- --------
                                            (IN THOUSANDS, EXCEPT PER SHARE DATA
<S>                                         <C>      <C>      <C>       <C>
Interest Income............................ $132,442 $127,020  $104,687  $98,494
Interest Expense...........................   58,589   57,677    45,819   43,566
                                            -------- -------- --------- --------
  NET INTEREST INCOME......................   73,853   69,343    58,868   54,928
   Provision for loan losses...............    5,175    4,220     3,100    3,875
  NET INTEREST INCOME AFTER PROVISION
   FOR LOAN LOSSES.........................   68,678   65,123    55,768   51,053
Noninterest Income.........................   11,997    9,147    11,717    8,986
Noninterest expense........................   42,398   40,276    32,592   31,198
                                            -------- -------- --------- --------
INCOME BEFORE INCOME TAXES.................   38,277   33,994    34,893   28,841
  Applicable income taxes..................   10,036    9,152    10,190    7,536
                                            -------- -------- --------- --------
NET INCOME................................. $ 28,241 $ 24,842 $  24,703 $ 21,305
                                            ======== ======== ========= ========
PER COMMON SHARE
  Net Income...............................    $2.00    $1.74     $1.75    $1.51
  Dividends................................     0.94     0.74      0.81     0.69
</TABLE>
 
                               S&T BANCORP, INC.
         UNAUDITED PRO FORMA CONDENSED CONSOLIDATED STATEMENT OF INCOME
                          YEAR ENDED DECEMBER 31, 1996
 
                     (IN THOUSANDS, EXCEPT PER SHARE DATA)
 
<TABLE>
<CAPTION>
                                                             SHARES PURCHASED AT
                                                             -------------------
                                                                $46       $49
                                                             PER SHARE PER SHARE
                                                             --------- ---------
<S>                                                          <C>       <C>
Interest Income (3)......................................... $131,917  $131,917
Interest Expense (3)........................................   61,981    62,240
                                                             --------  --------
  NET INTEREST INCOME.......................................   69,936    69,677
    Provision for loan losses...............................    5,175     5,175
  NET INTEREST INCOME AFTER PROVISION
   FOR LOAN LOSSES..........................................   64,761    64,502
Noninterest Income..........................................   11,997    11,997
Noninterest expense.........................................   42,398    42,398
                                                             --------  --------
INCOME BEFORE INCOME TAXES..................................   34,360    34,101
  Applicable income taxes (3)...............................    8,665     8,574
                                                             --------  --------
NET INCOME.................................................. $ 25,695  $ 25,527
                                                             ========  ========
PER COMMON SHARE
  Net Income................................................    $2.04     $2.02
  Dividends declared........................................     0.94      0.94
</TABLE>
 
                                       17
<PAGE>
 
                               S&T BANCORP, INC.
         UNAUDITED PRO FORMA CONDENSED CONSOLIDATED STATEMENT OF INCOME
                      NINE MONTHS ENDED SEPTEMBER 30, 1997
 
                     (IN THOUSANDS, EXCEPT PER SHARE DATA)
 
<TABLE>
<CAPTION>
                                                             SHARES PURCHASED AT
                                                             -------------------
                                                                $46       $49
                                                             PER SHARE PER SHARE
                                                             --------- ---------
<S>                                                          <C>       <C>
Interest Income (3)......................................... $104,294  $104,294
Interest Expense (3)........................................   48,356    48,550
                                                             --------  --------
  NET INTEREST INCOME.......................................   55,938    55,744
    Provision for loan losses...............................    3,100     3,100
  NET INTEREST INCOME AFTER PROVISION
   FOR LOAN LOSSES..........................................   52,838    52,644
Noninterest Income..........................................   11,717    11,717
Noninterest expense.........................................   32,592    32,592
                                                             --------  --------
INCOME BEFORE INCOME TAXES..................................   31,963    31,769
  Applicable income taxes (3)...............................    9,164     9,097
                                                             --------  --------
NET INCOME.................................................. $ 22,799  $ 22,672
                                                             ========  ========
PER COMMON SHARE
  Net Income................................................    $1.81     $1.80
  Dividends declared........................................     0.81      0.81
</TABLE>
 
                                       18
<PAGE>
 
                               S&T BANCORP, INC.
                      SELECTED HISTORICAL FINANCIAL RATIOS
 
<TABLE>
<CAPTION>
                                             YEAR ENDED         NINE MONTHS
                                            DECEMBER 31,    ENDED SEPTEMBER 30,
                                            --------------  -------------------
                                             1996    1995      1997      1996
                                            ------  ------  --------- ---------
<S>                                         <C>     <C>     <C>       <C>
SELECTED RATIOS:
FINANCIAL PERFORMANCE
Return on average assets...................  1.65%   1.53%      1.84%     1.68%
Return on average equity................... 13.01%  12.51%     13.82%    13.27%
CAPITAL
Leverage Ratio............................. 11.45%  11.42%     11.83%    11.46%
Tier I Risk Based Capital.................. 16.77%  16.83%     17.10%    16.93%
Total Risk Based Capital................... 18.01%  18.19%     18.36%    18.12%
Dividend payout ratio...................... 37.77%  35.25%     41.53%    37.62%
Shareholders' equity to total assets....... 12.65%  12.67%     13.65%    12.49%
Book value per share....................... $16.02  $14.99     $17.69    $15.47
ASSET QUALITY
Allowance for loan losses as a percent of
 loans.....................................   1.56%   1.55%     1.62%     1.60%
Allowance for loan losses as a percent of
 nonperforming loans.......................    182%    359%      502%      506%
Nonperforming loans to total loans.........   0.86%   0.43%     0.32%     0.32%
</TABLE>
 
                               S&T BANCORP, INC.
                 UNAUDITED SELECTED PRO FORMA FINANCIAL RATIOS
                               DECEMBER 31, 1996
 
<TABLE>
<CAPTION>
                                                             SHARES PURCHASED AT
                                                             -------------------
                                                                $46       $49
                                                             PER SHARE PER SHARE
                                                             --------- ---------
<S>                                                          <C>       <C>
SELECTED RATIOS:
FINANCIAL PERFORMANCE
Return on average equity....................................  17.49%    17.94%
CAPITAL
Leverage Ratio..............................................   7.53%     7.27%
Tier I Risk Based Capital...................................  11.11%    10.73%
Total Risk Based Capital....................................  12.50%    12.12%
Dividend payout ratio.......................................  44.16%    44.45%
Shareholders' equity to total assets........................   8.84%     8.59%
Book value per share........................................  $12.46    $12.10
</TABLE>
 
 
                                       19
<PAGE>
 
                               S&T BANCORP, INC.
                 UNAUDITED SELECTED PRO FORMA FINANCIAL RATIOS
                              SEPTEMBER 30, 1997
 
<TABLE>
<CAPTION>
                                                             SHARES PURCHASED AT
                                                             -------------------
                                                                $46       $49
                                                             PER SHARE PER SHARE
                                                             --------- ---------
<S>                                                          <C>       <C>
SELECTED RATIOS:
FINANCIAL PERFORMANCE
Return on average equity....................................  18.01%    18.42%
CAPITAL
Leverage Ratio..............................................   8.04%     7.79%
Tier I Risk Based Capital...................................  11.70%    11.34%
Total Risk Based Capital....................................  12.96%    12.60%
Dividend payout ratio.......................................  43.22%    43.46%
Shareholders' equity to total assets........................   9.94%     9.69%
Book value per share........................................  $14.33    $13.98
</TABLE>
 
                               S&T BANCORP, INC.
              NOTES TO UNAUDITED PRO FORMA FINANCIAL INFORMATION
 
(1) The pro forma financial information reflects the repurchase of 1,500,000
    Shares at $46.00 and $49.00 per share, as appropriate.
 
(2) The balance sheet data give effect to the purchase of shares as of the
    balance sheet date. The income statement data give effect to the purchase
    of shares as of the beginning of each period presented.
 
(3) The funds used to purchase shares were considered to have been proceeds
    from long term borrowings and the sale of securities available for sale.
    The pro forma data assumes a rate of interest of 5.75% on the borrowings,
    a rate of interest of 5.25% on the securities and a statutory tax rate of
    35%.
 
(4) No effect has been given to the $250,000 in costs estimated to be incurred
    in connection with the Purchase Offer. Such costs are not expected to be
    material and will be capitalized as part of the cost of the stock
    purchased.
 
                                      20
<PAGE>
 
                        10. SOURCE AND AMOUNT OF FUNDS
 
  Assuming that the Company purchases 1,500,000 Shares pursuant to the Offer
at a price of $49.00 per Share, the total amount required by the Company to
purchase such Shares will be $73,500,000, exclusive of fees and other
expenses. The Company will fund such purchase with dividends from its
subsidiaries, S&T Bank and S&T Investment Company. Each such subsidiary has
funds available to fund the proposed transaction through securities available
for sale, cash, or borrowings made in the ordinary course of business. Any
borrowings would be repaid out of normal cash flows.
 
      11. INTEREST OF DIRECTORS AND EXECUTIVE OFFICERS; TRANSACTIONS AND
                        ARRANGEMENTS CONCERNING SHARES
 
  As of January 15, 1998, the Company had issued and outstanding 14,143,655
Shares, and had reserved 542,000 Shares for issuance upon exercise of
currently exercisable stock options. The 1,500,000 Shares that the Company is
offering to purchase represent approximately 11% of the outstanding Shares. As
of January 15, 1998, the Company's directors and executive officers as a group
(27 persons) beneficially owned an aggregate of 1,250,196 Shares (including
465,000 shares covered by currently exercisable options granted under the
Company's Incentive Stock Plan) representing approximately 9% of the
outstanding Shares, assuming the exercise of the options by such persons. As
of January 15, 1998, the Thrift Plan held 521,715 shares, representing
approximately 4% of the shares outstanding.
 
  Except as set forth in Schedule A, neither the Company, nor any subsidiary
of the Company nor, to the best of the Company's knowledge, any of the
Company's directors or executive officers, nor any affiliates of any of the
foregoing, had any transactions involving the Shares during the 40 business
days prior to the date hereof.
 
  Executive officers and directors of the Company may participate in the Offer
on the same basis as the Company's other shareholders. The Company has not
been advised whether any executive officer or director intends to tender any
Shares pursuant to the Offer or at what price such tenders, if any, would be
made. The table below identifies each executive officer and director of the
Company and sets forth the number of Shares owned (including Shares that could
be acquired upon the exercise of currently exercisable stock options and
Shares held under the Thrift Plan). Subject to the terms of the Offer, all or
a portion of such Shares could be tendered. In addition, such executive
officers and directors may acquire additional Shares prior to the Expiration
Date as a result of participation in the Dividend Reinvestment Plan.
 
<TABLE>
<CAPTION>
                          SHARES   STOCK
NAME                       OWNED  OPTIONS  TOTAL
- ----                      ------- ------- -------
<S>                       <C>     <C>     <C>
DIRECTORS
Brice, Thomas A.           48,598  7,500   56,098
Brubaker, Forrest L.       31,121  7,500   38,621
Carino, James L.           92,815  7,500  100,315
Delaney, John J.           32,036  7,500   39,536
Duggan, Robert D.          24,767 94,000  118,767
Garges, Thomas W.           1,460  7,500    8,960
Gatti, William             11,040  7,500   18,540
Grant, Ruth M.            104,448      0  104,448
Grube, Jeffrey D.           1,006      0    1,006
Hanna, Herbert L.         125,440  7,500  132,940
Jones, Frank W.             8,557      0    8,557
Kirk, Joseph A.            18,811  7,500   26,311
Levy, Samuel               66,624  7,500   74,124
Papernick, Alan             1,000      0    1,000
Ruddock, Judge W. Parker   24,280  7,500   31,780
Sampson, Myles D.           3,643      0    3,643
Spadafora, Charles A.      25,897  7,500   33,397
Toretti, Christine J.     104,793  7,500  112,293
</TABLE>
 
                                      21
<PAGE>
 
<TABLE>
<CAPTION>
                     SHARES   STOCK
NAME                  OWNED  OPTIONS   TOTAL
- ----                 ------- ------- ---------
<S>                  <C>     <C>     <C>
EXECUTIVE OFFICERS
Barone, James G.       5,086  36,000    41,086
Hauck, Edward C.       3,111  22,000    25,111
Klumpp, H. William     6,409  16,000    22,409
Krieger, David L.      7,299  36,000    43,299
Miller, James C.      19,543  60,000    79,543
Onderick, Edward A.    4,022  25,000    29,022
Rout, Robert E.        4,940  23,000    27,940
Salome, Bruce W.       4,908  36,000    40,908
Smead, J. Jeffrey      3,542  27,000    30,542
                     ------- ------- ---------
TOTAL                785,196 465,000 1,250,196
                     ======= ======= =========
</TABLE>
 
  Decisions as to whether to tender Shares held in the Thrift Plan will be
made as described under "Section 3--Procedure for Tendering Shares."
 
  Except for outstanding options to purchase Shares granted to certain
employees (including executive officers) of the Company, and except as
otherwise described herein, neither the Company nor, to the best of the
Company's knowledge, any of its affiliates, directors or executive officers,
or any of the directors or executive officers of any of its affiliates, is a
party to any contract, arrangement, understanding or relationship with any
other person relating, directly or indirectly, to the Offer with respect to
any securities of the Company including, but not limited to, any contract,
arrangement, understanding or relationship concerning the transfer or the
voting of any such securities, joint ventures, loan or option arrangements,
puts or calls, guaranties of loans, guaranties against loss or the giving or
withholding of proxies, consents or authorizations.
 
  Except as disclosed in this Offer, the Company has no plans or proposals
which relate to or would result in: (a) the acquisition by any person of
additional securities of the Company or the disposition of securities of the
Company; (b) an extraordinary corporate transaction, such as a merger,
reorganization or liquidation, involving
the Company or any of its subsidiaries; (c) a sale or transfer of a material
amount of assets of the Company or any of its subsidiaries; (d) any change in
the present Board of Directors or management of the Company; (e) any material
change in the present dividend rate or policy, or indebtedness or
capitalization of the Company; (f) any other material change in the Company's
corporate structure or business; (g) any change in the Company's Certificate
of Incorporation or Bylaws or any actions which may impede the acquisition of
control of the Company by any person; (h) a class of equity security of the
Company being delisted from a national securities exchange; (i) a class of
equity securities of the Company becoming eligible for termination of
registration pursuant to Section 12(g)(4) of the Exchange Act; or (j) the
suspension of the Company's obligation to file reports pursuant to Section
15(d) of the Exchange Act.
 
   12. EFFECTS OF THE OFFER ON THE MARKET FOR SHARES; REGISTRATION UNDER THE
                                 EXCHANGE ACT
 
  The Company's purchase of Shares pursuant to the Offer will reduce the
number of Shares that might otherwise be traded publicly and may reduce the
number of shareholders. Nonetheless, the Company anticipates that there will
be a sufficient number of Shares outstanding and publicly traded following
consummation of the Offer to ensure a continued trading market for the Shares.
Based upon published guidelines of the Nasdaq National Market, the Company
believes that following its purchase of Shares pursuant to the Offer, the
Company's remaining Shares will continue to qualify to be quoted on the Nasdaq
National Market.
 
  The Shares are currently "margin securities" under the rules of The Board of
Governors of the Federal Reserve System (the "Federal Reserve Board"). This
has the effect, among other things, of allowing brokers to extend credit to
their customers using such Shares as collateral. The Company believes that,
following the purchase of Shares pursuant to the Offer, the Shares will
continue to be "margin securities" for purposes of the Federal Reserve Board's
margin regulations.
 
 
                                      22
<PAGE>
 
  The Shares are registered under the Exchange Act, which requires, among
other things, that the Company furnish certain information to its shareholders
and the U.S. Securities and Exchange Commission (the "Commission") and comply
with the Commission's proxy rules in connection with meetings of the Company's
shareholders.
 
                13. CERTAIN LEGAL MATTERS; REGULATORY APPROVALS
 
  The Company is not aware of any license or regulatory permit that appears to
be material to the Company's business that might be adversely affected by the
Company's acquisition of Shares as contemplated herein or of any approval or
other action by, or any filing with, any government or governmental,
administrative or regulatory authority or agency, domestic or foreign, that
would be required for the acquisition or ownership of Shares by the Company as
contemplated herein. Should any such approval or other action be required, the
Company presently contemplates that such approval or other action will be
sought. The Company is unable to predict whether it may determine that it is
required to delay the acceptance for payment of or payment for Shares tendered
pursuant to the Offer pending the outcome of any such matter. There can be no
assurance that any such approval or other action, if needed, would be obtained
or would be obtained without substantial conditions or that the failure to
obtain any such approval or other action might not result in adverse
consequences to the Company's business. The Company's obligations under the
Offer to accept for payment and pay for Shares is subject to certain
conditions. See Section 6.
 
                  14. CERTAIN FEDERAL INCOME TAX CONSEQUENCES
 
  In General. The following is a discussion of the material United States
federal income tax consequences to shareholders with respect to a sale of
Shares pursuant to the Offer. The discussion is based upon the provisions of
the Internal Revenue Code of 1986, as amended (the "Code"), U.S. Department of
Treasury regulations, Internal Revenue Service ("IRS") rulings and judicial
decisions, all in effect as of the date hereof and all of which are subject to
change (possibly with retroactive effect) by subsequent legislative, judicial
or administrative action. The discussion does not address all aspects of
United States federal income taxation that may be relevant to a particular
shareholder in light of such shareholder's particular circumstances or to
certain types of holders subject to special treatment under the United States
federal income tax laws (such as certain financial institutions, tax-exempt
organizations, life insurance companies, dealers in securities or currencies,
employee benefit plans or shareholders holding the Shares as part of a
conversion transaction, as part of a hedge or hedging transaction, or as a
position in a straddle for tax purposes). In addition, the discussion below
does not consider the effect of any foreign, state, local or other tax laws
that may be applicable to particular shareholders. The discussion assumes that
the Shares are held as "capital assets" within the meaning of Section 1221 of
the Code. The Company has neither requested nor obtained a written opinion of
counsel or a ruling from the IRS with respect to the tax matters discussed
below.
 
  EACH SHAREHOLDER SHOULD CONSULT HIS OR HER OWN TAX ADVISOR AS TO THE
PARTICULAR UNITED STATES FEDERAL INCOME TAX CONSEQUENCES TO THAT SHAREHOLDER
OF TENDERING SHARES PURSUANT TO THE OFFER AND THE APPLICABILITY AND EFFECT OF
ANY STATE, LOCAL OR FOREIGN TAX LAWS AND RECENT CHANGES IN APPLICABLE TAX
LAWS.
 
  Characterization of the Surrender of Shares Pursuant to the Offer. The
surrender of Shares by a shareholder to the Company pursuant to the Offer will
be a taxable transaction for United States federal income tax purposes. The
United States federal income tax consequences to a shareholder may vary
depending upon the shareholder's particular facts and circumstances. Under
Section 302 of the Code, the surrender of Shares by a shareholder to the
Company pursuant to the Offer will be treated as a "sale or exchange" of such
Shares for United States federal income tax purposes (rather than as a
distribution by the Company with respect to the Shares held by the tendering
shareholder) if the receipt of cash upon such surrender (i) is "substantially
disproportionate" with respect to the shareholder, (ii) results in a "complete
redemption" of the shareholder's
 
                                      23
<PAGE>
 
interest in the Company, or (iii) is "not essentially equivalent to a
dividend" with respect to the shareholder (each as described below).
 
  If any of the above three tests is satisfied, and the surrender of the
Shares is therefore treated as a "sale or exchange" of such Shares for United
States federal income tax purposes, the tendering shareholder will recognize
gain or loss equal to the difference between the amount of cash received by
the shareholder and the shareholder's tax basis in the Shares surrendered
pursuant to the Offer. Any such gain or loss will be capital gain or loss, and
will be long term capital gain or loss if the Shares have been held for more
than one year.
 
  If none of the above three tests is satisfied, the tendering shareholder
will be treated as having received a distribution by the Company with respect
to such shareholder's Shares in an amount equal to the cash received by the
shareholder pursuant to the Offer. The distribution will be treated as a
dividend taxable as ordinary income to the extent of the Company's current or
accumulated earnings and profits for tax purposes. The amount of the
distribution in excess of the Company's current or accumulated earnings and
profits will be treated as a return of the shareholder's tax basis in the
Shares, and then as gain from the sale or exchange of such Shares. If a
shareholder is treated as having received a distribution by the Company with
respect to his or her Shares, the shareholder's tax basis in his or her
remaining Shares will generally be adjusted to take into account the
shareholder's return of basis in the Shares tendered.
 
  Constructive Ownership. In determining whether any of the three tests under
Section 302 of the Code is satisfied, shareholders must take into account not
only the Shares that are actually owned by the shareholder, but also Shares
that are constructively owned by the shareholder within the meaning of Section
318 of the Code. Under Section 318 of the Code, a shareholder may
constructively own Shares actually owned, and in some cases constructively
owned, by certain related individuals or entities and Shares that the
shareholder has the right to acquire by exercise of an option or by
conversion.
 
  Proration. Contemporaneous dispositions or acquisitions of Shares by a
shareholder or related individuals or entities may be deemed to be part of a
single integrated transaction and may be taken into account in determining
whether any of the three tests under Section 302 of the Code has been
satisfied. Each shareholder should be aware that because proration may occur
in the Offer, even if all the Shares actually and constructively owned by a
shareholder are tendered pursuant to the Offer, fewer than all of such Shares
may be purchased by the Company. Thus, proration may affect whether the
surrender by a shareholder pursuant to the Offer will meet any of the three
tests under Section 302 of the Code.
 
  Section 302 Tests. The receipt of cash by a shareholder will be
"substantially disproportionate" if the percentage of the outstanding Shares
in the Company actually and constructively owned by the shareholder
immediately following the surrender of Shares pursuant to the Offer is less
than 80% of the percentage of the outstanding Shares actually and
constructively owned by such shareholder immediately before the sale of Shares
pursuant to the Offer. Shareholders should consult their tax advisors with
respect to the application of the "substantially disproportionate" test to
their particular situation.
 
  The receipt of cash by a shareholder will be a "complete redemption" if
either (i) the shareholder owns no Shares in the Company either actually or
constructively immediately after the Shares are surrendered pursuant to the
Offer, or (ii) the shareholder actually owns no Shares in the Company
immediately after the surrender of Shares pursuant to the Offer and, with
respect to Shares constructively owned by the shareholder immediately after
the Offer, the shareholder is eligible to waive (and effectively waives)
constructive ownership of all such Shares under procedures described in
Section 302(c) of the Code. A director, officer or employee of the Company is
not eligible to waive constructive ownership under the procedures described in
Section 302(c) of the Code.
 
  Even if the receipt of cash by a shareholder fails to satisfy the
"substantially disproportionate" test or the "complete redemption" test, a
shareholder may nevertheless satisfy the "not essentially equivalent to a
dividend" test if the shareholder's surrender of Shares pursuant to the Offer
results in a "meaningful reduction" in the shareholder's interest in the
Company. Whether the receipt of cash by a shareholder will be "not
 
                                      24
<PAGE>
 
essentially equivalent to a dividend" will depend upon the individual
shareholder's facts and circumstances. The IRS has indicated in published
rulings that even a small reduction in the proportionate interest of a small
minority shareholder in a publicly held corporation who exercises no control
over corporate affairs may constitute such a "meaningful reduction."
Shareholders expecting to rely upon the "not essentially equivalent to a
dividend" test should consult their own tax advisors as to its application in
their particular situation.
 
  Corporate Shareholder Dividend Treatment. If a sale of Shares by a corporate
shareholder is treated as a dividend, the corporate shareholder may be
entitled to claim a deduction equal to 70% of the dividend under Section 243
of the Code, subject to applicable limitations. Corporate shareholders should,
however, consider the effect of Section 246(c) of the Code, which disallows
the 70% dividends received deduction with respect to stock that is held for 45
days or less. For this purpose, the length of time a taxpayer is deemed to
have held stock may be reduced by periods during which the taxpayer's risk of
loss with respect the stock is diminished by reason of the existence of
certain options or other transactions. Moreover, under Section 246A of the
Code, if a corporate shareholder has incurred indebtedness directly
attributable to an investment in Shares, the 70% dividends-received deduction
may be reduced.
 
  In addition, amounts received by a corporate shareholder pursuant to the
Offer that are treated as a dividend may constitute an "extraordinary
dividend" under Section 1059 of the Code. The "extraordinary dividend" rules
of the Code are highly complicated. Accordingly, any corporate shareholder
that might have a dividend as a result of the sale of shares pursuant to the
Offer should review the "extraordinary dividend" rules to determine the
applicability and impact of such rules to it.
 
  Additional Tax Considerations. The distinction between long-term capital
gains and ordinary income is relevant because, in general, individuals
currently are subject to taxation at a reduced rate on their "net capital
gain" (i.e., the excess of net long-term capital gains over net short-term
capital losses) for the year. Tax rates on long-term capital gain for
individual shareholders vary depending on the shareholder's income and holding
period for the Shares. In particular, different maximum tax rates apply to
gains recognized by an individual from the sale of (i) assets held for more
than one year but not more than 18 months (currently 28 percent), and (ii)
assets held for more than 18 months (currently 20 percent).
 
  Shareholders are urged to consult their own tax advisors regarding any
possible impact on their obligation to make estimated tax payments as a result
of the recognition of any capital gain (or the receipt of any ordinary income)
caused by the surrender of any Shares to the Company pursuant to the Offer.
 
  Foreign Shareholders. The Company will withhold United States federal income
tax at a rate of 30% from gross proceeds paid pursuant to the Offer to a
foreign shareholder or his agent, unless the Company determines that a reduced
rate of withholding is applicable pursuant to a tax treaty or that an
exemption from withholding is applicable because such gross proceeds are
effectively connected with the conduct of a trade or business by the foreign
shareholder within the United States. For this purpose, a foreign shareholder
is any shareholder that is not (i) a citizen or resident of the United States,
(ii) a domestic corporation or domestic partnership, (iii) an estate the
income of which from sources without the United States is effectively
connected with the conduct of a trade or business within the United States, or
(iv) a trust if a court within the United States is able to exercise primary
supervision over the administration of the trust, and one or more United
States persons have the authority to control all substantial decisions of the
trust. Without definite knowledge to the contrary, the Company will determine
whether a shareholder is a foreign shareholder by reference to the
shareholder's address. A foreign shareholder may be eligible to file for a
refund of such tax or a portion of such tax if such shareholder (i) meets the
"complete redemption," "substantially disproportionate" or "not essentially
equivalent to a dividend" tests described above, (ii) is entitled to a reduced
rate of withholding pursuant to a treaty and the Company withheld at a higher
rate, or (iii) is otherwise able to establish that no tax or a reduced amount
of tax was due. In order to claim an exemption from withholding on the ground
that gross proceeds paid pursuant to the Offer are effectively connected with
the conduct of a trade or business by a foreign shareholder within the United
States or that the foreign shareholder is entitled to the benefits of a tax
treaty, the foreign shareholder must deliver to the
 
                                      25
<PAGE>
 
Depositary (or other person who is otherwise required to withhold United
States tax) a properly executed statement claiming such exemption or benefits.
Such statements may be obtained from the Depositary. Foreign shareholders are
urged to consult their own tax advisors regarding the application of United
States federal income tax withholding, including eligibility for a withholding
tax reduction or exemption and the refund procedures.
 
  Backup Withholding. See Section 3 with respect to the application of the
United States federal income tax backup withholding.
 
  THE TAX DISCUSSION SET FORTH ABOVE IS INCLUDED FOR GENERAL INFORMATION ONLY
AND MAY NOT APPLY TO SHARES ACQUIRED IN CONNECTION WITH THE EXERCISE OF STOCK
OPTIONS OR PURSUANT TO OTHER COMPENSATION ARRANGEMENTS WITH THE COMPANY. THE
TAX CONSEQUENCES OF A SALE PURSUANT TO THE OFFER MAY VARY DEPENDING UPON,
AMONG OTHER THINGS, THE PARTICULAR CIRCUMSTANCES OF THE TENDERING SHAREHOLDER.
NO INFORMATION IS PROVIDED HEREIN AS TO THE STATE, LOCAL OR FOREIGN TAX
CONSEQUENCES OF THE TRANSACTION CONTEMPLATED BY THE OFFER. SHAREHOLDERS ARE
URGED TO CONSULT THEIR OWN TAX ADVISORS TO DETERMINE THE PARTICULAR FEDERAL,
STATE, LOCAL AND FOREIGN TAX CONSEQUENCES TO THEM OF TENDERING SHARES PURSUANT
TO THE OFFER AND THE EFFECT OF THE STOCK OWNERSHIP ATTRIBUTION RULES DESCRIBED
ABOVE.
 
            15. EXTENSION OF TENDER PERIOD; TERMINATION; AMENDMENTS
 
  The Company expressly reserves the right, in its sole discretion and at any
time or from time to time, to extend the period of time during which the Offer
is open by giving oral or written notice of such extension to the Depositary
and making a public announcement thereof. There can be no assurance, however,
that the Company will excise its right to extend the Offer. During any such
extension, all Shares previously tendered will remain subject to the Offer,
except to the extent that such Shares may be withdrawn as set forth in Section
4. The Company also expressly reserves the right, in its sole discretion, (i)
to terminate the Offer and not accept for payment any Shares not theretofore
accepted for payment or, subject to Rule 13e-4(f)(5) under the Exchange Act,
which requires the Company either to pay the consideration offered or to
return the Shares tendered promptly after the termination or withdrawal of the
Offer, to postpone payment for Shares upon the occurrence of any of the
conditions specified in Section 6 hereof, by giving oral or written notice of
such termination to the Depositary and making a public announcement thereof
and (ii) at any time, or from time to time, to amend the Offer in any respect.
Amendments to the Offer may be effected by public announcement. Without
limiting the manner in which the Company may choose to make public
announcement of any extension, termination or amendment, the Company shall
have no obligation (except as otherwise required by applicable law) to
publish, advertise or otherwise communicate any such public announcement,
other than by making a release to the Dow Jones News Service, except in the
case of an announcement of an extension of the Offer, in which case the
Company shall have no obligation to publish, advertise or otherwise
communicate such announcement other than by issuing a notice of such extension
by press release or other public announcement, which notice shall be issued no
later than 9:00 a.m., Eastern time, on the next business day after the
previously scheduled Expiration Date. Material changes to information
previously provided to holders of the Shares in this Offer or in documents
furnished subsequent thereto will be disseminated to holders of Shares in
compliance with Rule 13e-4(e)(2) promulgated by the Commission under the
Exchange Act.
 
  If the Company materially changes the terms of the Offer or the information
concerning the Offer, or if it waives a material condition of the Offer, the
Company will extend the Offer to the extent required by Rules 13e-4(d)(2) and
13e-4(e)(2) under the Exchange Act. Those rules require that the minimum
period during which an offer must remain open following material changes in
the terms of the offer or information concerning the offer (other than a
change in price, change in dealer's soliciting fee or change in percentage of
securities sought) will depend on the facts and circumstances, including the
relative materiality of such terms or information. In a published release, the
Commission has stated that in its view, an offer should remain open for a
minimum of five
 
                                      26
<PAGE>
 
business days from the date that notice of such a material change is first
published, sent or given. The Offer will continue or be extended for at least
ten business days from the time the Company publishes, sends or gives to
holders of Shares a notice that it will (a) increase or decrease the price it
will pay for Shares or the amount of the Dealer Manager's soliciting fee or
(b) increase (except for an increase not exceeding 2% of the outstanding
shares) or decrease the number of Shares it seeks.
 
                             16. FEES AND EXPENSES
 
  Keefe, Bruyette & Woods, Inc. ("KBW") will act as Dealer Manager for the
Company in connection with the Offer. The Company has agreed to pay the Dealer
Manager, upon acceptance for and payment of Shares pursuant to the Offer, a
total of $0.05 per Share purchased by the Company pursuant to the Offer. The
Dealer Manager will also be indemnified against certain liabilities, including
liabilities under the federal securities laws, in connection with the Offer.
 
  The Company has retained ChaseMellon Shareholder Services, L.L.C. as
Depositary and Corporate Investor Communications, Inc., as Information Agent
in connection with the Offer. The Information Agent may contact shareholders
by mail, telephone, facsimile, telex, telegraph, or other electronic means and
personal interviews, and may request brokers, dealers and other nominee
shareholders to forward materials relating to the Offer to beneficial owners.
The Depositary and the Information Agent will receive reasonable and customary
compensation for their services and will also be reimbursed for certain out-
of-pocket expenses. The Company has agreed to indemnify the Depositary and the
Information Agent against certain liabilities, including certain liabilities
under the federal securities laws, in connection with the Offer. Neither the
Information Agent nor the Depositary has been retained to make solicitations
or recommendations in connection with the Offer.
 
  The Company will not pay any fees or commissions to any broker, dealer or
other person for soliciting tenders of Shares pursuant to the Offer (other
than the fee of the Dealer Manager). The Company will, upon request, reimburse
brokers, dealers, commercial banks and trust companies for reasonable and
customary handling and mailing expenses incurred by them in forwarding
materials relating to the Offer to their customers.
 
                          17. ADDITIONAL INFORMATION
 
  The Company is subject to the informational requirements of the Exchange Act
and, in accordance therewith, files reports, proxy statements and other
information with the Commission. Such reports, proxy statements and other
information may be inspected and copied at the public reference facilities
maintained by the Commission at Room 1024, 450 Fifth Street, N.W., Washington,
D.C. 20549 and at the Commission's regional offices at 7 World Trade Center,
13th Floor, Suite 1300, New York, New York 10048 and Suite 1400, Citicorp
Center, 500 West Madison Street, Chicago, Illinois 60661. Copies of such
material may also be obtained by mail from the Public Reference Section of the
Commission at 450 Fifth Street, N.W., Washington, D.C. 20549 at prescribed
rates. Such information may also be accessed electronically by means of the
Commission's home page on the Internet (http://www.sec.gov).
 
                               18. MISCELLANEOUS
 
  Pursuant to Rule 13e-4 under the Exchange Act, the Company has filed with
the Commission an Issuer Tender Offer Statement on Schedule 13e-4 which
contains additional information with respect to the Offer. Such Schedule 13e-
4, including the exhibits and any amendments thereto, may be examined, and
copies may be obtained, at the same places and in the same manner as is set
forth in Section 17 with respect to information concerning the Company.
 
  The Offer is being made to all holders of Shares. The Company is not aware
of any state where the making of the Offer is prohibited by administrative or
judicial action pursuant to a valid state statute. If the Company becomes
aware of any valid state statute prohibiting the making of the Offer, the
Company will make a good faith effort to comply with such statute. If, after
such good faith effort, the Company cannot comply with such
 
                                      27
<PAGE>
 
statute, the Offer will not be made to, nor will tenders be accepted from or
on behalf of, holders of Shares in such state. In those jurisdictions whose
securities, blue sky or other laws require the Offer to be made by a licensed
broker or dealer, the Offer shall be deemed to be made on behalf of the
Company by the Dealer Manager or one or more registered brokers or dealers
licensed under the laws of such jurisdictions.
 
                                          S&T BANCORP, INC.
 
January 20, 1998
 
                                      28
<PAGE>
 
                                  SCHEDULE A
 
          CERTAIN TRANSACTIONS INVOLVING SHARES BY THE COMPANY OR ITS
                        EXECUTIVE OFFICERS OR DIRECTORS
 
  During the 40 business days prior to January 20, 1998, the only transactions
effected in the Shares by the Company, its executive officers or directors
were as follows:
 
<TABLE>
<CAPTION>
                        NUMBER OF    ACQUISITION
INDIVIDUAL           SHARES ACQUIRED    PRICE     NATURE OF TRANSACTION
- ----------           --------------- ----------- -----------------------
<S>                  <C>             <C>         <C>
Barone, James G.           200         $43.25    Thrift Plan Allocations
Duggan, Robert D.          216         $43.25    Thrift Plan Allocations
Hauck, Edward C.           234         $43.25    Thrift Plan Allocations
Klumpp, H. William         161         $43.25    Thrift Plan Allocations
Krieger, David L.          216         $43.25    Thrift Plan Allocations
Miller, James C.           216         $43.25    Thrift Plan Allocations
Onderick, Edward A.        148         $43.25    Thrift Plan Allocations
Rout, Robert E.            215         $43.25    Thrift Plan Allocations
Salome, Bruce W.           211         $43.25    Thrift Plan Allocations
Smead, J. Jeffrey          172         $43.25    Thrift Plan Allocations
</TABLE>
 
  All transactions effected in the Company stock during the 40 business days
prior to January 20, 1998 were normal, recurring Thrift Plan allocations due
to salary deferrals, employer match and year end profit sharing contributions.
 
                                      29
<PAGE>
 
                     The Dealer Manager for the Offer is:
 
                         KEEFE, BRUYETTE & WOODS, INC.
                            Two World Trade Center
                           New York, New York 10048
                                (212) 323-8450
 
  Any questions concerning the terms of the Offer may be directed to the
Dealer Manager.
 
                    The Information Agent for the Offer is:
 
                    CORPORATE INVESTOR COMMUNICATIONS, INC.
                               111 Commerce Road
                           Carlstadt, NJ 07022-2586
 
                     Banks and Brokers call (800) 346-7885
                   All others call Toll Free (800) 201-9605
 
  Any questions concerning tender procedures or requests for additional copies
of this Offer to Purchase, the Letter of Transmittal or other tender offer
materials may be directed to the Information Agent.
 
                       The Depositary for the Offer is:
 
                   CHASEMELLON SHAREHOLDER SERVICES, L.L.C.
 
            BY MAIL:                                OVERNIGHT DELIVERY:
 
 
    P.O. Box 3301                                85 Challenger Road
    South Hackensack, NJ 07606                   Mail Drop Reorg
                                                 Ridgefield Park, NJ 07660
                                                 Attn: Reorganization Dept.
 
                                   BY HAND:
 
                               120 Broadway
                               13th Floor
                               New York, NY 10271
 
Telephone number: (888) 884-8091
Facsimile number: (201) 329-8936
Facsimile confirmation number (for eligible institutions only): (201) 296-4860
 
                               January 20, 1998
 
                                      30
<PAGE>
 
                                                                  Exhibit (a)(2)
                               S&T BANCORP, INC.
                             LETTER OF TRANSMITTAL
 
                      TO ACCOMPANY SHARES OF COMMON STOCK
 
                                       OF
 
                               S&T BANCORP, INC.
 
                   TENDERED PURSUANT TO THE OFFER TO PURCHASE
                             DATED JANUARY 20, 1998
 
 THE OFFER, PRORATION PERIOD AND WITHDRAWAL RIGHTS WILL EXPIRE AT 5:00
 P.M., EASTERN TIME, ON FEBRUARY 18, 1998, UNLESS THE OFFER IS EXTENDED.
 
                  To: CHASEMELLON SHAREHOLDER SERVICES, L.L.C.
 
             By Mail:                          Overnight Delivery:
 
           P.O. Box 3301                       85 Challenger Road
    South Hackensack, NJ 07606                   Mail Drop Reorg
                                            Ridgefield Park, NJ 07660
                                           Attn: Reorganization Dept.
 
                                    By Hand:
 
                                  120 Broadway
                                   13th Floor
                               New York, NY 10271
                        Telephone number: (888) 884-8091
 
                        Facsimile number: (201) 329-8936
 Facsimile confirmation number (for eligible institutions only): (201) 296-4860
 
 
           DESCRIPTION OF SHARES TENDERED (SEE INSTRUCTIONS 3 AND 4)
 
- -----------------------------------------------------------------------------

      NAME(S) AND ADDRESS(ES)
      OF REGISTERED HOLDER(S)
     (PLEASE FILL IN EXACTLY AS                     SHARES TENDERED
NAME(S) APPEAR(S) ON CERTIFICATE(S))   (ATTACH ADDITIONAL LIST IF NECESSARY)
- -----------------------------------------------------------------------------
                                                    TOTAL NUMBER
                                                      OF SHARES
                                                     REPRESENTED   NUMBER OF
                                      CERTIFICATE        BY          SHARES
                                         NUMBER    CERTIFICATE(S)* TENDERED**
                                      ---------------------------------------
                                      ---------------------------------------
                                      ---------------------------------------
                                      ---------------------------------------
                                      ---------------------------------------
                                      TOTAL SHARES
- -----------------------------------------------------------------------------
  * Need not be completed by shareholders tendering by book-entry
    transfer.
 
 ** Unless otherwise indicated, it will be assumed that all Shares
    represented by any certificates delivered to the Depositary are
    being tendered. See Instruction 4.
 
<PAGE>
 
  DELIVERY OF THIS INSTRUMENT TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE OR
TRANSMISSION OF INSTRUCTIONS VIA A FACSIMILE NUMBER OTHER THAN ONE LISTED
ABOVE WILL NOT CONSTITUTE A VALID DELIVERY.
 
  THE INSTRUCTIONS ACCOMPANYING THIS LETTER OF TRANSMITTAL SHOULD BE READ
CAREFULLY BEFORE THE LETTER OF TRANSMITTAL IS COMPLETED.
 
  Delivery of documents to S&T Bancorp, Inc. or to the Book-Entry Transfer
Facility does not constitute a valid delivery. PLEASE DO NOT MAIL OR DELIVER
ANY SHARES TO S&T BANCORP. DELIVERIES TO S&T BANCORP, INC. WILL NOT BE
FORWARDED TO THE DEPOSITARY AND THEREFORE WILL NOT CONSTITUTE VALID DELIVERY.
 
               (BOX BELOW FOR USE BY ELIGIBLE INSTITUTIONS ONLY)
 
 [_]CHECK HERE IF TENDERED SHARES ARE BEING DELIVERED BY BOOK-ENTRY TRANSFER
    TO THE DEPOSITARY'S ACCOUNT AT THE DEPOSITARY TRUST COMPANY ("DTC") AND
    COMPLETE THE FOLLOWING:
 
 Name of Tendering Institution ______________________________________________
 
 Account No. _________________________________________________________________
 
 Transaction Code No. ________________________________________________________
 
 [_]CHECK HERE IF CERTIFICATES FOR TENDERED SHARES ARE BEING DELIVERED
    PURSUANT TO A NOTICE OF GUARANTEED DELIVERY PREVIOUSLY SENT TO THE
    DEPOSITARY AND COMPLETE THE FOLLOWING:
 
   Name(s) of Registered Owner(s): ___________________________________________
 
   Date of Execution of Notice of Guaranteed Delivery: _______________________
 
   Name of Institution Which Guaranteed Delivery: ____________________________
 
   Check Box and Give Account Number and Transaction Code if Delivered by
    Book-Entry Transfer:
 
     [_] DTC
 
   Account Number:____________________________________________________________
 
   Transaction Code Number:___________________________________________________
 
 
                                   ODD LOTS
                              (SEE INSTRUCTION 9)
 
   This section is to be completed ONLY if Shares are being tendered by or on
 behalf of a person owning beneficially, as of the close of business on
 January 15, 1998, and who continues to own beneficially as of the Expiration
 Date, an aggregate of fewer than 100 Shares.
 
   The undersigned either (check one box):
 
   [_] was the beneficial owner as of the close of business on January 15,
   1998, and continues to be the beneficial owner as of the Expiration
   Date, of an aggregate of fewer than 100 Shares, all of which are being
   tendered, or
 
   [_] is a broker, dealer, commercial bank, trust company or other
   nominee that (i) is tendering, for the beneficial owners thereof,
   Shares with respect to which it is the record owner, and (ii) believes,
   based upon representations made to it by each such beneficial owner,
   that such beneficial owner owned beneficially as of the close of
   business on January 15, 1998, and continues to own beneficially as of
   the Expiration Date, an aggregate of fewer than 100 Shares, and is
   tendering all of such Shares.
 
<PAGE>
 
                   NOTE: SIGNATURES MUST BE PROVIDED BELOW.
 
              PLEASE READ THE ACCOMPANYING INSTRUCTIONS CAREFULLY
 
LADIES AND GENTLEMEN:
 
  The undersigned hereby tenders to S&T Bancorp, Inc., a Pennsylvania
corporation (the "Company"), the above-described shares of its Common Stock,
par value $2.50 per share (the "Shares"), at a price per Share hereinafter set
forth, pursuant to the Company's offer to purchase up to 1,500,000 Shares,
upon the terms and subject to the conditions set forth in the Offer to
Purchase, dated January 20, 1998 (the "Offer to Purchase"), receipt of which
is hereby acknowledged, and in this Letter of Transmittal (which together
constitute the "Offer").
 
  Subject to, and effective upon, acceptance for payment of and payment for
the Shares tendered herewith in accordance with the terms and subject to the
conditions of the Offer (including, if the Offer is extended or amended, the
terms and conditions of any such extension or amendment), the undersigned
hereby sells, assigns and transfers to, or upon the order of, the Company all
right, title and interest in and to all the Shares that are being tendered
hereby or orders the registration of such Shares tendered by book-entry
transfer that are purchased pursuant to the Offer to or upon the order of the
Company and irrevocably constitutes and appoints the Depositary the true and
lawful agent and attorney-in-fact of the undersigned with respect to such
Shares, with full power of substitution (such power of attorney being deemed
to be an irrevocable power coupled with an interest), to (a) deliver
certificates for such Shares, or transfer ownership of such Shares on the
account books maintained by any of the Book-Entry Transfer Facilities,
together, in any such case, with all accompanying evidences of transfer and
authenticity, to or upon the order of the Company upon receipt by the
Depositary, as the undersigned's agent, of the Purchase Price (as defined
below) with respect to such Shares, (b) present certificates for such Shares
for cancellation and transfer on the books of the Company and (c) receive all
benefits and otherwise exercise all rights of beneficial ownership of such
Shares, all in accordance with the terms of the Offer.
 
  The undersigned hereby represents and warrants that the undersigned has full
power and authority to tender, sell, assign and transfer the Shares tendered
hereby and that, when and to the extent the same are accepted for payment by
the Company, the Company will acquire good, marketable and unencumbered title
thereto, free and clear of all liens, restrictions, charges, encumbrances,
conditional sales agreements or other obligations relating to the sale or
transfer thereof, and the same will not be subject to any adverse claims. The
undersigned will, upon request, execute and deliver any additional documents
deemed by the Depositary or the Company to be necessary or desirable to
complete the sale, assignment and transfer of the Shares tendered hereby.
 
  The undersigned hereby represents and warrants that the undersigned has read
and agrees to all of the terms of the Offer. All authority herein conferred or
agreed to be conferred shall not be affected by, and shall survive the death
or incapacity of the undersigned, and any obligation of the undersigned
hereunder shall be binding upon the heirs, personal representatives,
successors and assigns of the undersigned. Except as stated in the Offer, this
tender is irrevocable.
 
  The undersigned understands that tenders of Shares pursuant to any one of
the procedures described in Section 2 or 3 of the Offer to Purchase and in the
Instructions hereto will constitute the undersigned's acceptance of the terms
and conditions of the Offer, including the undersigned's representation and
warranty that (i) the undersigned has a net long position in the Shares being
tendered within the meaning of Rule 14e-4 promulgated under the Securities
Exchange Act of 1934, as amended, and (ii) the tender of such Shares complies
with Rule 14e-4. The Company's acceptance for payment of Shares tendered
pursuant to the Offer will constitute a binding agreement between the
undersigned and the Company upon the terms and subject to the conditions of
the Offer. The undersigned understands that the Company will determine a
single per Share price (not less than $46.00 nor in excess of $49.00 per
Share) net to the seller in cash, without interest thereon, (the "Purchase
Price") that it will pay for Shares validly tendered and not withdrawn
pursuant to the Offer taking into account the number of Shares so tendered and
the prices specified by tendering shareholders. The undersigned understands
that the Company will select the Purchase Price that will allow it to purchase
1,500,000 Shares (or such lesser number of Shares as are validly tendered and
not withdrawn at prices not less than $46.00 nor in excess of
<PAGE>
 
$49.00 per Share) pursuant to the Offer. The undersigned understands that all
Shares properly tendered and not withdrawn at prices at or below the Purchase
Price will be purchased at the Purchase Price, net to the seller in cash,
without interest thereon, upon the terms and subject to the conditions of the
Offer, including its proration provisions, and that the Company will return
all other Shares, including Shares tendered and not withdrawn at prices
greater than the Purchase Price and Shares not purchased because of proration.
The undersigned understands that tenders of Shares pursuant to any one of the
procedures described in Section 2 or 3 of the Offer to Purchase and in the
instructions hereto will constitute an agreement between the undersigned and
the Company upon the terms and subject to the conditions of the Offer.
 
  The undersigned recognizes that, under certain circumstances set forth in
the Offer to Purchase, the Company may terminate or amend the Offer or may
postpone the acceptance for payment of, or the payment for, Shares tendered or
may not be required to purchase any of the Shares tendered hereby or may
accept for payment fewer than all of the Shares tendered hereby.
 
  Unless otherwise indicated under "Special Payment Instructions," please
issue the check for the purchase price of any Shares purchased, and/or return
any Shares not tendered or not purchased, in the name(s) of the undersigned
(and, in the case of Shares tendered by book-entry transfer, by credit to the
account at the Book-Entry Transfer Facility). Similarly, unless otherwise
indicated under "Special Delivery Instructions," please mail the check for the
purchase price of any Shares purchased and/or any certificates for Shares not
tendered or not purchased (and accompanying documents, as appropriate) to the
undersigned at the address shown below the undersigned's signature(s). In the
event that both "Special Payment Instructions" and "Special Delivery
Instructions" are completed, please issue the check for the purchase price of
any Shares purchased and/or return any Shares not tendered or not purchased in
the name(s) of, and mail said check and/or any certificates to, the person(s)
so indicated. The undersigned recognizes that the Company has no obligation,
pursuant to the "Special Payment Instructions," to transfer any Shares from
the name of the registered holder(s) thereof if the Company does not accept
for payment any of the Shares so tendered.
 
 
                         PRICE (IN DOLLARS) PER SHARE
                      AT WHICH SHARES ARE BEING TENDERED
                              (SEE INSTRUCTION 5)
 
     CHECK ONLY ONE BOX. IF MORE THAN ONE BOX IS CHECKED, OR IF NO BOX IS
       CHECKED (EXCEPT AS PROVIDED IN THE ODD LOTS BOX AND INSTRUCTIONS
                  BELOW), THERE IS NO VALID TENDER OF SHARES.
 
    [_] $46.00 [_] $46.50 [_] $47.00 [_] $47.50
                                          [_] $48.00 [_] $48.50 [_] $49.00
 
<PAGE>
 
 
                   S&T BANCORP, INC. DIVIDEND REINVESTMENT
                          PLAN (SEE INSTRUCTION 13)
 
   This section is to be completed ONLY if Shares held in the Company's
 Dividend Reinvestment Plan are to be tendered.
 
   [_] By checking this box, the undersigned represents that the undersigned
       is a participant in the S&T Bancorp, Inc. Dividend Reinvestment Plan
       and hereby tenders the following number of Shares held in the
       Dividend Reinvestment Plan account of the undersigned at ChaseMellon
       Shareholder Service, L.L.C.:

       _______________ Shares*
 
     * The undersigned understands and agrees that all Shares held in the
       Dividend Reinvestment Plan account of the undersigned at ChaseMellon
       Shareholder Services, L.L.C. will be tendered if the above box is
       checked and the space above is left blank.
 
<PAGE>
 
 
                                   IMPORTANT

 (Please Complete Substitute Form W-9 Included in this Letter of Transmittal)
 
- -------------------------------------------------------------------------------
                          (SIGNATURE(S) OF OWNER(S))
- -------------------------------------------------------------------------------
 
- -------------------------------------------------------------------------------
                            (PLEASE PRINT NAME(S))
 
 
Dated                    , 1998 Capacity (full title)
     --------------------                            --------------------------
                                               
 
Address
       ----------------------------------------------------------------------
                              (INCLUDE ZIP CODE)
 
Area Code and Telephone No.
                           ----------------------------------------------------
 
Tax Identification or Social Security No.
                                         --------------------------------------
                                               (see Substitute Form W-9)
 
(Must be signed by registered holder(s) exactly as name(s) appear(s) on stock
certificate(s) or on a security position listing or by person(s) authorized to
become registered holder(s) by certificates and documents transmitted
herewith.
 
If signature is by a trustee, executor, administrator, guardian, attorney-in-
fact, officer of a corporation or other person acting in a fiduciary or
representative capacity, please set forth full title and see Instruction 6.)
 
                           GUARANTEE OF SIGNATURE(S)
                          (SEE INSTRUCTIONS 1 AND 6)
 
Authorized Signature(s)
                       --------------------------------------------------------
 
Name of Firm
            -------------------------------------------------------------------
                                     (PLEASE PRINT)
 
Name and Title
              -----------------------------------------------------------------
                                     (PLEASE PRINT)
 
Address
       ------------------------------------------------------------------------
                                 (INCLUDE ZIP CODE)
 
Area Code and Telephone Number
                              -------------------------------------------------
 
Dated                                       , 1998
     ---------------------------------------         
<PAGE>
 
 
 
   SPECIAL PAYMENT INSTRUCTIONS              SPECIAL DELIVERY INSTRUCTIONS
   (SEE INSTRUCTIONS 6, 7 AND 8)             (SEE INSTRUCTIONS 6, 7 AND 8)
  
 To be completed ONLY if the                To be completed ONLY if the
 check for the purchase price of            check for the purchase price of
 Shares purchased and/or                    Shares purchased and/or
 certificates for Shares not                certificates for Shares not
 tendered or not purchased are to           tendered or not purchased are to
 be issued in the name of someone           be mailed to someone other than
 other than the undersigned.                the undersigned or to the
                                            undersigned at an address other
                                            than that shown below the
                                            undersigned's signature(s).
 
 Issue  [_] check and/or
 [_] certificate(s) to:
 
 Name ____________________________
          (PLEASE PRINT)
 
                                            Mail  [_] check and/or
                                            [_] certificate(s) to:
 
 Address _________________________
 
 _________________________________          Name ____________________________
        (INCLUDE ZIP CODE)                              (PLEASE PRINT)
 
        
 
                                            Address _________________________
 
 _________________________________
    (TAXPAYER IDENTIFICATION OR             _________________________________
       SOCIAL SECURITY NO.)                         (INCLUDE ZIP CODE)
 
                                                                     
 
 IF SPECIAL PAYMENT INSTRUCTIONS            IF SPECIAL DELIVERY INSTRUCTIONS
 ARE BEING GIVEN, PLEASE REMEMBER           ARE BEING GIVEN, PLEASE REMEMBER
 TO HAVE YOUR SIGNATURE                     TO HAVE YOUR SIGNATURE         
 GUARANTEED.                                GUARANTEED                      
                                            
<PAGE>
 
                                 INSTRUCTIONS
             FORMING PART OF THE TERMS AND CONDITIONS OF THE OFFER
 
  1. Guarantee of Signatures. Except as otherwise provided below, all
signatures on this Letter of Transmittal must be guaranteed by a firm that is
a member of a registered national securities exchange or the National
Association of Securities Dealers, Inc., or by a commercial bank, a trust
company, a savings bank, a savings and loan association or a credit union
which has membership in an approved Signature Guarantee Medallion Program (an
"Eligible Institution"). SIGNATURES ON THIS LETTER OF TRANSMITTAL NEED NOT BE
GUARANTEED (A) IF THIS LETTER OF TRANSMITTAL IS SIGNED BY THE REGISTERED
HOLDER(S) OF THE SHARES (which term, for purposes of this document, shall
include any participant in one of the Book-Entry Transfer Facilities whose
name appears on a security position listing as the owner of Shares) tendered
herewith AND SUCH HOLDER(S) HAVE NOT COMPLETED THE BOX ENTITLED "SPECIAL
PAYMENT INSTRUCTIONS" OR THE BOX ENTITLED "SPECIAL DELIVERY INSTRUCTIONS" ON
THIS LETTER OF TRANSMITTAL or (B) if such Shares are tendered for the account
of an Eligible Institution. See Instruction 6.
 
  2. Delivery of Letter of Transmittal and Shares. This Letter of Transmittal
or, in the case of a book-entry transfer, an Agent's Message (as defined
below), is to be used either if certificates are to be forwarded herewith or
if delivery of Shares is to be made by book-entry transfer pursuant to the
procedures set forth in Section 3 of the Offer to Purchase. CERTIFICATES FOR
ALL PHYSICALLY DELIVERED SHARES, OR A CONFIRMATION OF A BOOK-ENTRY TRANSFER
INTO THE DEPOSITARY'S ACCOUNT AT THE BOOK-ENTRY TRANSFER FACILITY OF ALL
SHARES DELIVERED ELECTRONICALLY, AS WELL AS A PROPERLY COMPLETED AND DULY
EXECUTED LETTER OF TRANSMITTAL (OR MANUALLY SIGNED COPY THEREOF) AND ANY OTHER
DOCUMENTS REQUIRED BY THIS LETTER OF TRANSMITTAL, MUST BE RECEIVED BY THE
DEPOSITARY AT ITS ADDRESS AS SET FORTH ON THE FRONT PAGE OF THIS LETTER OF
TRANSMITTAL ON OR PRIOR TO THE EXPIRATION DATE (as defined in the Offer to
Purchase). The term "Agent's Message" means a message transmitted by a Book-
Entry Transfer Facility to, and received by, the Depositary and forming a part
of a Book-Entry confirmation, which states that such Book-Entry Transfer
Facility has received an express acknowledgement from the participant in such
Book-Entry Transfer Facility tendering the Shares, that such participant has
received and agrees to be bound by the terms of the Offer to Purchase and the
Letter of Transmittal and that the Company may enforce such agreement against
the participant.
 
  THE METHOD OF DELIVERY OF THIS LETTER OF TRANSMITTAL, SHARE CERTIFICATES AND
ALL OTHER REQUIRED DOCUMENTS IS AT THE OPTION AND RISK OF THE TENDERING
SHAREHOLDER, AND DELIVERY WILL BE DEEMED MADE ONLY WHEN ACTUALLY RECEIVED BY
THE DEPOSITARY. IF CERTIFICATES FOR SHARES ARE SENT BY MAIL, REGISTERED MAIL
WITH RETURN RECEIPT REQUESTED, PROPERLY INSURED, IS RECOMMENDED. DO NOT MAIL
OR DELIVER TO S&T BANCORP.
 
  No alternative or contingent tenders will be accepted. See Section 1 of the
Offer to Purchase. By executing this Letter of Transmittal (or facsimile
thereof), the tendering shareholder waives any right to receive any notice of
the acceptance for payment of the Shares.
 
  3. Inadequate Space. If the space provided herein is inadequate, the
certificate numbers and/or the number of Shares should be listed on a separate
signed schedule attached hereto.
 
  4. Partial Tenders (Not Applicable to Shareholders Who Tender By Book-Entry
Transfer). If fewer than all the Shares represented by any certificate
delivered to the Depositary are to be tendered, fill in the number of Shares
that are to be tendered in the box entitled "Number of Shares Tendered." In
such case, a new certificate for the remainder of the Shares represented by
the old certificate will be sent to the person(s) signing this Letter of
Transmittal, unless otherwise provided in the "Special Payment Instructions"
or "Special Delivery Instructions" boxes on this Letter of Transmittal, as
promptly as practicable following the expiration or termination of the Offer.
All Shares represented by certificates delivered to the Depositary will be
deemed to have been tendered unless otherwise indicated.
<PAGE>
 
  5. Indication of Price at Which Shares Are Being Tendered. FOR SHARES TO BE
VALIDLY TENDERED, THE SHAREHOLDER MUST CHECK THE BOX INDICATING THE PRICE PER
SHARE AT WHICH HE OR SHE IS TENDERING SHARES UNDER "PRICE (IN DOLLARS) PER
SHARE AT WHICH SHARES ARE BEING TENDERED" IN THIS LETTER OF TRANSMITTAL,
except that any shareholder who owned beneficially as of the close of business
on January 15, 1998, and continues to own beneficially as of the Expiration
Date, an aggregate of fewer than 100 Shares, may check the box above in the
section entitled "Odd Lots" indicating that such shareholder is tendering all
Shares at the Purchase Price determined by the Company. Only one box may be
checked. IF MORE THAN ONE BOX IS CHECKED OR IF NO BOX IS CHECKED, THERE IS NO
VALID TENDER OF SHARES. A shareholder wishing to tender portions of his or her
Share holdings at different prices must complete a separate Letter of
Transmittal for each price at which he or she wishes to tender each such
portion of his or her Shares. The same Shares cannot be tendered (unless
previously validly withdrawn as provided in Section 4 of the Offer to
Purchase) at more than one price.
 
  6. Signatures on Letter of Transmittal; Stock Powers and Endorsements. If
this Letter of Transmittal is signed by the registered holder(s) of the Shares
hereby, the signature(s) must correspond with the name(s) as written on the
face of the certificates without alteration, enlargement or any change
whatsoever.
 
  If any of the Shares hereby is held of record by two or more persons, all
such persons must sign this Letter of Transmittal.
 
  If any of the Shares tendered hereby are registered in different names on
different certificates, it will be necessary to complete, sign and submit as
many separate Letters of Transmittal as there are different registrations of
certificates.
 
  IF THIS LETTER OF TRANSMITTAL IS SIGNED BY THE REGISTERED HOLDER(S) OF THE
SHARES TENDERED HEREBY, NO ENDORSEMENTS OF CERTIFICATES OR SEPARATE STOCK
POWERS ARE REQUIRED UNLESS PAYMENT OF THE PURCHASE PRICE IS TO BE MADE TO, OR
SHARES NOT TENDERED OR NOT PURCHASED ARE TO BE REGISTERED IN THE NAME OF, ANY
PERSON OTHER THAN THE REGISTERED HOLDER(S). SEE INSTRUCTION 1.
 
  If this Letter of Transmittal is signed by a person other than the
registered holder(s) of the Shares tendered hereby, certificates evidencing
the Shares tendered hereby must be endorsed or accompanied by appropriate
stock powers, in either case, signed exactly as the name(s) of the registered
holder(s) appear(s) on the certificates for such Shares. Signature(s) on any
such certificates or stock powers must be guaranteed by an Eligible
Institution. See Instruction 1.
 
  If this Letter of Transmittal or any certificate or stock power is signed by
a trustee, executor, administrator, guardian, attorney-in-fact, officer of a
corporation or other person acting in a fiduciary or representative capacity,
such person should so indicate when signing, and proper evidence satisfactory
to the Company of the authority of such person so to act must be submitted.
 
  7. Stock Transfer Taxes. The Company will pay or cause to be paid any stock
transfer taxes with respect to the sale and transfer of any Shares to it or
its order pursuant to the Offer. If, however, payment of the purchase price is
to be made to, or Shares not tendered or not purchased are to be registered in
the name of, any person other than the registered holder(s), or if tendered
Shares are registered in the name of any person other than the person(s)
signing this Letter of Transmittal, the amount of any stock transfer taxes
(whether imposed on the registered holder(s), such other person or otherwise)
payable on account of the transfer to such person will be deducted from the
purchase price unless satisfactory evidence of the payment of such taxes, or
exemption therefrom, is submitted. See Section 5 of the Offer to Purchase.
EXCEPT AS PROVIDED IN THIS INSTRUCTION 7, IT WILL NOT BE NECESSARY TO AFFIX
TRANSFER TAX STAMPS TO THE CERTIFICATES REPRESENTING SHARES TENDERED HEREBY.
 
  8. Special Payment and Delivery Instructions. If the check for the purchase
price of any Shares purchased is to be issued in the name of, and/or any
Shares not tendered or not purchased are to be returned to, a person
<PAGE>
 
other than the person(s) signing this Letter of Transmittal or if the check
and/or any certificates for Shares not tendered or not purchased are to be
mailed to someone other than the person(s) signing this Letter of Transmittal
or to an address other than that shown above in the box captioned "Description
of Shares Tendered," then the boxes captioned "Special Payment Instructions"
and/or "Special Delivery Instructions" on this Letter of Transmittal should be
completed. Shareholders tendering Shares by book-entry transfer will have any
Shares not accepted for payment returned by crediting the account maintained
by such shareholder at the Book-Entry Transfer Facility from which such
transfer was made.
 
  9. Odd Lots. As described in the Offer to Purchase, if fewer than all Shares
validly tendered at or below the Purchase Price and not withdrawn on or prior
to the Expiration Date are to be purchased, the Shares purchased first will
consist of all Shares tendered by any shareholder who owned beneficially as of
the close of business on January 15, 1998, and continues to own beneficially
as of the Expiration Date, an aggregate of fewer than 100 Shares and who
validly tendered all such Shares at or below the Purchase Price. Partial
tenders of Shares will not qualify for this preference. This preference will
not be available unless the box captioned "Odd Lots" in this Letter of
Transmittal is completed.
 
  10. Substitute Form W-9 and Form W-8. THE TENDERING SHAREHOLDER IS REQUIRED
TO PROVIDE THE DEPOSITARY WITH EITHER A CORRECT TAXPAYER IDENTIFICATION NUMBER
("TIN") ON SUBSTITUTE FORM W-9, WHICH IS PROVIDED UNDER "IMPORTANT TAX
INFORMATION" BELOW, OR A PROPERLY COMPLETED FORM W-8. FAILURE TO PROVIDE THE
INFORMATION ON EITHER SUBSTITUTE FORM W-9 OR FORM W-8 MAY SUBJECT THE
TENDERING SHAREHOLDER TO 31% FEDERAL INCOME TAX BACKUP WITHHOLDING ON THE
PAYMENT OF THE PURCHASE PRICE. The box in Part 3 of Substitute Form W-9 may be
checked if the tendering shareholder has not been issued a TIN and has applied
for a number or intends to apply for a number in the near future. If the box
in Part 3 is checked and the Depositary is not provided with a TIN by the time
of payment, the Depositary will withhold 31% on all payments of the purchase
price thereafter until a TIN is provided to the Depositary.
 
  11. Requests for Assistance or Additional Copies. Any questions or requests
for assistance may be directed to the Information Agent or the Dealer Manager
at their respective telephone numbers and addresses listed below. Requests for
additional copies of the Offer to Purchase, this Letter of Transmittal or
other tender offer materials may be directed to the Information Agent or the
Dealer Manager and such copies will be furnished promptly at the Company's
expense. Shareholders may also contact their local broker, dealer, commercial
bank or trust company for assistance concerning the Offer.
 
  12. Irregularities. All questions as to the Purchase Price, the form of
documents, and the validity, eligibility (including time of receipt) and
acceptance of any tender of Shares will be determined by the Company, in its
sole discretion, and its determination shall be final and binding. The Company
reserves the absolute right to reject any or all tenders of Shares that it
determines are not in proper form or the acceptance for payment of or payment
for Shares that may, in the opinion of the Company's counsel, be unlawful.
Except as otherwise provided in the Offer to Purchase, the Company also
reserves the absolute right to waive any of the conditions to the Offer or any
defect or irregularity in any tender of Shares and the Company's
interpretation of the terms and conditions of the Offer (including these
instructions) shall be final and binding. Unless waived, any defects or
irregularities in connection with tenders must be cured within such time as
the Company shall determine. None of the Company, the Dealer Manager, the
Depositary, the Information Agent or any other person shall be under any duty
to give notice of any defect or irregularity in tenders, nor shall any of them
incur any liability for failure to give any such notice. Tenders will not be
deemed to have been made until all defects and irregularities have been cured
or waived.
 
  13. Dividend Reinvestment Plan. Shareholders who participate in the
Company's Dividend Reinvestment Plan who want to tender Shares held under that
plan pursuant to the Offer should mark the box under "S&T BANCORP, INC.
DIVIDEND REINVESTMENT PLAN" and indicate the number of Shares that are to be
tendered. If such box is marked but the number of Shares to be tendered is not
indicated, all Shares held for the
<PAGE>
 
shareholder's account in the Company will be tendered. All Shares held for the
shareholders' account in the Company's Dividend Reinvestment Plan at
ChaseMellon Shareholder Services, L.L.C. will be tendered pursuant to the same
price listed under "PRICE (IN DOLLARS) PER SHARE AT WHICH SHARES ARE BEING
TENDERED."
 
  IMPORTANT: THIS LETTER OF TRANSMITTAL (OR A MANUALLY SIGNED COPY THEREOF)
TOGETHER WITH CERTIFICATES OR CONFIRMATION OF BOOK-ENTRY TRANSFER (AND ALL
OTHER REQUIRED DOCUMENTS) OR THE NOTICE OF GUARANTEED DELIVERY MUST BE
RECEIVED BY THE DEPOSITARY, ON OR PRIOR TO THE EXPIRATION DATE.
 
                           IMPORTANT TAX INFORMATION
 
  Under federal income tax law, a shareholder whose tendered Shares are
accepted for payment is required to provide the Depositary (as payer) with
such shareholder's correct TIN on Substitute Form W-9 below. If such
shareholder is an individual, the TIN is his or her social security number.
For businesses and other entities, the number is the employer identification
number. If the Depositary is not provided with the correct TIN or properly
completed Form W-8, the shareholder may be subject to a $50 penalty imposed by
the Internal Revenue Service. In addition, payments that are made to such
shareholder with respect to Shares purchased pursuant to the Offer may be
subject to backup withholding.
 
  Certain shareholders (including, among others, all corporations and certain
foreign individuals and entities) are not subject to these backup withholding
and reporting requirements. In order for a noncorporate foreign shareholder to
qualify as an exempt recipient, that shareholder must complete and sign a Form
W-8, Certificate of Foreign Status, attesting to that shareholder's exempt
status. The Form W-8 can be obtained from the Depositary. Exempt shareholders,
other than noncorporate foreign shareholders, should furnish their TIN in Part
1 and check the box in Part 4 of the Substitute Form W-9 below and sign, date
and return the Substitute Form W-9 to the Depositary. See the enclosed
Guidelines for Certification of Taxpayer Identification Number on Substitute
Form W-9 for additional instructions.
 
  If federal income tax backup withholding applies, the Depositary is required
to withhold 31% of any payments made to the shareholder. Backup withholding is
not an additional tax. Rather, the federal income tax liability of persons
subject to backup withholding will be reduced by the amount of the tax
withheld. If withholding results in an overpayment of taxes, a refund may be
obtained.
 
PURPOSE OF SUBSTITUTE FORM W-9 AND FORM W-8
 
  To avoid backup withholding on payments that are made to a shareholder with
respect to Shares purchased pursuant to the Offer, the shareholder is required
to notify the Depositary of his or her correct TIN by completing the
Substitute Form W-9 included in this Letter of Transmittal certifying that the
TIN provided on Substitute Form W-9 is correct and that (1) the shareholder
has not been notified by the Internal Revenue Service that he or she is
subject to federal income tax backup withholding as a result of failure to
report all interest or dividends or (2) the Internal Revenue Service has
notified the shareholder that he or she is no longer subject to federal income
tax backup withholding. Foreign shareholders must submit a properly completed
Form W-8 in order to avoid the applicable backup withholding; provided,
however, that backup withholding will not apply to foreign shareholders
subject to 30% (or lower treaty rate) withholding on gross payments received
pursuant to the Offer.
 
WHAT NUMBER TO GIVE THE DEPOSITARY
 
  The shareholder is required to give the Depositary the social security
number or employer identification number of the registered owner of the
Shares. If the Shares are in more than one name or are not in the name of the
actual owner, consult the enclosed Guidelines for Certification of Taxpayer
Identification Number on Substitute Form W-9 for additional guidance on which
number to report.
<PAGE>
 
(See Instruction 10)
Please fill in your name and address below.
 
 ................................................................................
                                      Name
 
 ................................................................................
                          Address (number and street)
 
 ................................................................................
                            City, State and Zip Code
 
             PAYER'S NAME: CHASEMELLON SHAREHOLDER SERVICES, L.L.C.
 
- --------------------------------------------------------------------------------
                      PART I--PLEASE PROVIDE YOUR
                      TIN IN THE BOX AT RIGHT AND
                      CERTIFY BY SIGNING AND DATING
                      BELOW.
 
                                                       -----------------------
 SUBSTITUTE                                             Social Security Number
 
 FORM W-9
                                                                 OR
 
                                                       -----------------------
                                                       Employer Identification
                                                                Number
- --------------------------------------------------------------------------------
 
    DEPARTMENT OF THE TREASURY INTERNAL REVENUE SERVICE
                      PART II--Certification--Under Penalties of Perjury, I
                      certify that:
                      (1) The number shown on this form is my correct
                      Taxpayer Identification Number (or I am waiting for a
                      number to be issued to me), (2) I am not subject to
                      backup withholding because (a) I am exempt from backup
                      withholding, or (b) I have not been notified by the
                      Internal Revenue Service ("IRS") that I am subject to
                      backup withholding as a result of failure to report
                      all interest or dividends or (c) the IRS has notified
                      me that I am no longer subject to backup withholding,
                      and (3) all other information provided on this form is
                      true, correct and complete.
 
- --------------------------------------------------------------------------------
 
 PAYER'S REQUEST FOR TAXPAYER IDENTIFICATION NUMBER (TIN)
                      PART III--
                      Awaiting
                      TIN [_]
 
- --------------------------------------------------------------------------------
 
 FOR PAYEE EXEMPT FROM BACKUP WITHHOLDING
                      PART IV--
                      Exempt [_]
 
                      Certificate Instructions--You must cross out Item (2)
                      in Part II above if you have been notified by the IRS
                      that you are currently subject to backup withholding
                      because of under reporting interest or dividends on
                      your tax return. However, if after being notified by
                      the IRS that you were subject to backup withholding,
                      you received another notification from the IRS stating
                      that you are no longer subject to backup withholding,
                      do not cross out Item (2). If you are exempt from
                      backup withholding, check the box in Part IV above.
 
                     ----------------------------------------------------------
 
                      Signature: _____________________ Date: __________________
 
                      Name (Please Print): ____________________________________
 
NOTE: NOTE: FAILURE TO COMPLETE AND RETURN THIS FORM MAY RESULT IN BACKUP
       WITHHOLDING OF 31% OF ANY PAYMENTS MADE TO YOU PURSUANT TO THE OFFER.
       PLEASE REVIEW THE ENCLOSED GUIDELINES FOR CERTIFICATION OF TAXPAYER
       IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9 FOR ADDITIONAL DETAILS.
 
YOU MUST COMPLETE THE FOLLOWING CERTIFICATE IF YOU CHECKED THE BOX IN PART 3 OF
THE SUBSTITUTE FORM W-9
<PAGE>
 
 
 
             CERTIFICATE OF AWAITING TAXPAYER IDENTIFICATION NUMBER
 
   I certify under penalties of perjury that a taxpayer identification
 number has not been issued to me, and either (1) I have mailed or
 delivered an application to receive a taxpayer identification number to
 the appropriate Internal Revenue Service Center or Social Security
 Administration Office or (2) I intend to mail or deliver an application in
 the near future. I understand that if I do not provide a taxpayer
 identification number to you by the time of payment, you are required to
 withhold 31% of all reportable payments thereafter made to me until I
 provide a number.
 
 -------------------------------------    ----------------------------------
               Signature                                  Date
 
<PAGE>
 
 
 
 
                      [This Page Intentionally Left Blank]
<PAGE>
 
 
 
 
                      [This Page Intentionally Left Blank]
<PAGE>
 
                             The Information Agent:
 
                    CORPORATE INVESTOR COMMUNICATIONS, INC.
                               111 Commerce Road
                              Carlstadt, NJ 07072
 
                            Banks and Brokers Call:
                                 (800) 346-7885
 
                           ALL OTHERS CALL TOLL FREE:
                                 (800) 201-9605
 
                              The Dealer Manager:
 
                         KEEFE, BRUYETTE & WOODS, INC.
                             Two World Trade Center
                               New York, NY 10048
                                 (212) 323-8450
<PAGE>
 
                                                                 Exhibit (a)(3)
              NOT VALID UNLESS SIGNED BY AN ELIGIBLE INSTITUTION.
 
                               S&T BANCORP, INC.
 
                         NOTICE OF GUARANTEED DELIVERY
                           OF SHARES OF COMMON STOCK
               OFFER TO PURCHASE FOR CASH UP TO 1,500,000 SHARES
                              OF ITS COMMON STOCK
                       AT A PURCHASE PRICE NOT LESS THAN
                   $46.00 NOR IN EXCESS OF $49.00 PER SHARE
 
  This form or a facsimile copy of it must be used to accept the Offer (as
defined below) if:
 
(a) certificates for common stock, par value $2.50 per share (the "Shares"),
    of S&T Bancorp, Inc., a Pennsylvania corporation, are not immediately
    available; or
 
(b) the procedure for book-entry transfer cannot be completed on a timely
    basis; or
 
(c) time will not permit the Letter of Transmittal or other required documents
    to reach the Depositary before the Expiration Date (as defined in Section
    1 of the Offer to Purchase, as defined below).
 
  This form or a facsimile of it, signed and properly completed, may be
delivered by hand, mail, telegram or facsimile transmission to the Depositary
by the Expiration Date. See "Section 3--Procedure for Tendering Shares" in the
Offer to Purchase.
 
                                  DEPOSITARY:
 
                 To: CHASEMELLON SHAREHOLDER SERVICES, L.L.C.
 
              By Mail:                             Overnight Delivery:
 
            P.O. Box 3301                          85 Challenger Road
     South Hackensack, NJ 07606                      Mail Drop Reorg
                                                Ridgefield Park, NJ 07660
                                               Attn: Reorganization Dept.
 
                                   By Hand:
 
                                 120 Broadway
                                  13th Floor
                              New York, NY 10271
                       Telephone number: (888) 884-8091
 
                       Facsimile number: (201) 329-8936
Facsimile confirmation number (for eligible institutions only): (201) 296-4860
 
  DELIVERY OF THIS INSTRUMENT TO AN ADDRESS OTHER THAN THOSE SHOWN ABOVE OR
TRANSMISSION OF INSTRUCTIONS TO A FACSIMILE NUMBER OTHER THAN THOSE LISTED
ABOVE DOES NOT CONSTITUTE A VALID DELIVERY.
<PAGE>
 
Ladies and Gentlemen:
 
  The undersigned hereby tenders to S&T Bancorp, Inc., at the price per Share
indicated below, net to the seller in cash, upon the terms and subject to the
conditions set forth in the Offer to Purchase, dated January 20, 1998 (the
"Offer to Purchase"), and the related Letter of Transmittal (which together
with the Offer to Purchase constitute the "Offer"), receipt of which is hereby
acknowledged,       Shares of common stock, par value $2.50 per share (the
"Shares"), pursuant to the guaranteed delivery procedure set forth under
"Section 3--Procedure for Tendering Shares" in the Offer to Purchase.
 
 
       PRICE (IN DOLLARS) PER SHARE AT WHICH SHARES ARE BEING TENDERED.
 
 By checking one of the boxes below, the undersigned hereby tenders Shares at
 the price checked.
 
 Price (in dollars) per Share at which Shares are being tendered:
 
                             [_] $46.00      [_] $48.00
                             [_] $46.50      [_] $48.50
                             [_] $47.00      [_] $49.00
                             [_] $47.50
 
<PAGE>
 
 
                                    ODD LOTS
 
   To be completed ONLY if Shares are being tendered by or on behalf of a
 person owning beneficially, as of the close of business on January 15,
 1998 and who continue to own beneficially as of the Expiration Date, an
 aggregate of fewer than 100 Shares.
 
   The undersigned either (check one):
 
   [_] was the beneficial owners, as of the close of business on January
       15, 1998 of an aggregate of fewer than 100 Shares, all of which are
       being tendered, or
 
   [_] is a broker, dealer, commercial bank, trust company or other nominee
       which
 
 (a) is tendering, for the beneficial owners thereof, Shares with respect
     to which it is the record owner, and
 
 (b) believes, based upon representations made to it by such beneficial
     owners, that each such person was the beneficial owner, as of the
     close of business on January 15, 1998, of an aggregate of fewer than
     100 Shares and is tendering all of such shares.
 
 ---------------------------------------------------------------------------
                        Certificate Nos. (if available):
 
 ---------------------------------------------------------------------------
 
 ---------------------------------------------------------------------------
 
 ---------------------------------------------------------------------------
 If Shares will be tendered by book-entry transfer, check box below:
 [_] The Depositary Trust Company
 
 Account Number: _______________________
 
 Name(s): __________________________________________________________________
                               PLEASE TYPE OR PRINT
 
 Address(es): ______________________________________________________________
 
 ___________________________________________________________________________
 
 Area Code and Telephone Number: ___________________________________________
 
 Sign Here: ________________________________________________________________
 
 Dated: ________________, 1998
 
<PAGE>
 
 
                                  GUARANTEE
 
                   (NOT TO BE USED FOR SIGNATURE GUARANTEE)
 
   The undersigned, a member firm of a registered national securities
 exchange, a member of the National Association of Securities Dealers, Inc.,
 or a commercial bank, trust company, savings association or credit union
 having an office or correspondent in the United States (each, an "Eligible
 Institution"), hereby (i) represents that the undersigned has a net long
 position in Shares in or equivalent securities within the meaning of Rule
 14e-4 promulgated under the Securities Exchange Act of 1934, as amended, at
 least equal to the shares tendered, (ii) represents that such tender of
 Shares complies with Rule 14e-4 and (iii) guarantees that either the
 certificates representing the Shares tendered hereby in proper form for
 transfer, or timely confirmation of the book-entry transfer of such Shares
 into the Depositary's account at The Depositary Trust Company (pursuant to
 the procedures set forth under "Section 3--Procedure for Tendering Shares"
 in the Offer to Purchase), together with a properly completed and duly
 executed Letter of Transmittal (or facsimile thereof) with any required
 signature guarantee and any other documents required by the Letter of
 Transmittal, will be received by the Depositary at one of its addresses set
 forth above within three Nasdaq trading days after the date of execution
 hereof.
 
 Name of Firm: ______________________     ------------------------------------
                                                  Authorized Signature
 
 Address: ___________________________     Name: ______________________________
 
 ____________________________________     Title: _____________________________
 
 Zip Code: __________________________
 
 Area Code and Telephone Number: ____
                                          Dated: _______________________, 1998
 
 DO NOT SEND SHARE CERTIFICATES WITH THIS NOTICE. SHARE CERTIFICATES SHOULD
 BE SENT WITH YOUR LETTER OF TRANSMITTAL.
 
<PAGE>
 
                                                                 Exhibit (a)(4)
                         KEEFE, BRUYETTE & WOODS, INC.
                            TWO WORLD TRADE CENTER
                              NEW YORK, NY 10048
                                (212) 323-8450
 
                               S&T BANCORP, INC.
 
                       OFFER TO PURCHASE FOR CASH UP TO
                     1,500,000 SHARES OF ITS COMMON STOCK
                   AT A PURCHASE PRICE NOT LESS THAN $46.00
                       NOR IN EXCESS OF $49.00 PER SHARE
 
          THE OFFER, PRORATION PERIOD AND WITHDRAWAL RIGHTS EXPIRE AT
                5:00 P.M., EASTERN TIME, ON FEBRUARY 18, 1998,
                         UNLESS THE OFFER IS EXTENDED.
 
To Brokers, Dealers, Commercial Banks,
 Trust Companies and Other Nominees:
 
  S&T Bancorp, Inc., a Pennsylvania corporation (the "Company"), has appointed
us to act as Dealer Manager in connection with its offer to purchase for cash
up to 1,500,000 shares of its Common Stock, $2.50 par value per share (the
"Shares"), at prices not less than $46.00 nor in excess of $49.00 per Share,
specified by shareholders tendering their Shares, upon the terms and subject
to the conditions set forth in the Company's Offer to Purchase, dated January
20, 1998, and in the related Letter of Transmittal (which together constitute
the "Offer").
 
  The Company will determine the single per Share price, not less than $46.00
nor in excess of $49.00 per Share, net to the seller in cash (the "Purchase
Price"), that it will pay for Shares validly tendered pursuant to the Offer,
taking into account the number of Shares so tendered and the prices specified
by tendering shareholders. The Company will select the Purchase Price that
will allow it to buy 1,500,000 Shares (or such lesser number of Shares as are
properly tendered at prices not less than $46.00 nor in excess of $49.00 per
Share). All Shares validly tendered at prices at or below the Purchase Price
and not withdrawn on or prior to the Expiration Date (as defined in Section 1
of the Offer to Purchase) will be purchased at the Purchase Price, subject to
the terms and conditions of the Offer, including the proration provisions. See
Section 1 of the Offer to Purchase.
 
  Upon the terms and subject to the conditions of the Offer, if, at the
expiration of the Offer, more than 1,500,000 Shares are validly tendered at or
below the Purchase Price and not withdrawn, the Company will buy Shares (i)
from shareholders who owned beneficially as of the close of business on
January 15, 1998, and continue to own beneficially as of the Expiration Date,
an aggregate of fewer than 100 Shares who properly tender all their Shares at
or below the Purchase Price, and (ii) then, on a pro rata basis, from all
other shareholders who properly tender their Shares at prices at or below the
Purchase Price (and do not withdraw them prior to the expiration of the
Offer). See Sections 1 and 2 of the Offer to Purchase. All Shares not
purchased pursuant to the Offer, including Shares tendered at prices greater
than the Purchase Price and Shares not purchased because of proration will be
returned to the tendering shareholders at the Company's expense as promptly as
practicable following the Expiration Date.
 
  THE OFFER IS NOT CONDITIONED ON ANY MINIMUM NUMBER OF SHARES BEING TENDERED
PURSUANT TO THE OFFER. SEE SECTION 6 OF THE OFFER TO PURCHASE.
 
  No fees or commissions will be payable to brokers, dealers or any person for
soliciting tenders of Shares pursuant to the Offer other than fees paid to the
Dealer Manager as described in the Offer to Purchase. The
 
Broker/Dealer Letter
<PAGE>
 
Company will, upon request, reimburse brokers and banks for reasonable and
customary handling and mailing expenses incurred by them in forwarding
materials relating to the Offer to their customers. The Company will pay all
stock transfer taxes applicable to its purchase of Shares pursuant to the
Offer, subject to Instruction 7 of the Letter of Transmittal.
 
  No broker, dealer, bank, trust company or fiduciary shall be deemed to be
the agent of the Company, Keefe, Bruyette & Woods, Inc. as "Dealer Manager,"
ChaseMellon Shareholder Services, L.L.C. as "Depositary," or Corporate
Investor Communications, Inc., as "Information Agent," for purposes of the
Offer.
 
  For your information and for forwarding to your clients for whom you hold
Shares registered in your name or in the name of your nominee, we are
enclosing the following documents:
 
  1. Offer to Purchase, dated January 20, 1998;
 
  2. Letter to Clients which may be sent to your clients for whose accounts
     you hold Shares registered in your name or in the name of your nominee,
     with space provided for obtaining such clients' instructions with regard
     to the Offer;
 
  3. The Notice of Guaranteed Delivery to be used to accept the Offer if
     shares and all other required documents cannot be delivered to the
     Depositary by the Expiration Date.
 
  4. Letter, dated January 20, 1998, from James C. Miller, President and
     Chief Executive Officer of the Company, to shareholders of the Company;
 
  5. Letter of Transmittal for your use and for the information of your
     clients (together with substitute Form W-9); and
 
  6. A return envelope addressed to ChaseMellon Shareholder Services, L.L.C.,
     as Depositary.
 
  WE URGE YOU TO CONTACT YOUR CLIENTS AS PROMPTLY AS POSSIBLE. THE OFFER,
PRORATION PERIOD AND WITHDRAWAL RIGHTS WILL EXPIRE AT 5:00 P.M., EASTERN TIME,
ON FEBRUARY 18, 1998, UNLESS THE OFFER IS EXTENDED.
 
  In order to take advantage of the Offer, a duly executed and properly
completed Letter of Transmittal and any other required documents should be
sent to the Depositary with either certificate(s) representing the tendered
Shares or confirmation of their book-entry transfer, all in accordance with
the instructions set forth in the Letter of Transmittal and the Offer to
Purchase.
 
  Any inquiries you may have with respect to the Offer should be addressed to
the Depositary, the Information Agent or the Dealer Manager at their
respective addresses and telephone numbers set forth on the back cover page of
the Offer to Purchase.
 
  Additional copies of the enclosed material may be obtained from the
Information Agent, telephone: Banks and Brokers call (800) 346-7855, all
others call (800) 201-9605.
 
                                          Very truly yours,
 
                                          KEEFE, BRUYETTE & WOODS, INC.
 
Enclosures
 
  NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL CONSTITUTE YOU
OR ANY OTHER PERSON AS AN AGENT OF THE COMPANY OR ANY OF ITS AFFILIATES, THE
DEALER MANAGER OR THE DEPOSITARY, OR AUTHORIZE YOU OR ANY OTHER PERSON TO USE
ANY DOCUMENT OR MAKE ANY STATEMENT ON BEHALF OF ANY OF THEM IN CONNECTION WITH
THE OFFER OTHER THAN THE DOCUMENTS ENCLOSED HEREWITH AND THE STATEMENTS
CONTAINED THEREIN.
<PAGE>
 
                                                                 Exhibit (a)(5)
                               S&T BANCORP, INC.
 
                       OFFER TO PURCHASE FOR CASH UP TO
                     1,500,000 SHARES OF ITS COMMON STOCK
                   AT A PURCHASE PRICE NOT LESS THAN $46.00
                       NOR IN EXCESS OF $49.00 PER SHARE
 
          THE OFFER, PRORATION PERIOD AND WITHDRAWAL RIGHTS EXPIRE AT
                5:00 P.M., EASTERN TIME, ON FEBRUARY 18, 1998,
                         UNLESS THE OFFER IS EXTENDED.
 
To Our Clients:
 
  Enclosed for your consideration are the Offer to Purchase, dated January 20,
1998, and the related Letter of Transmittal (which together constitute the
"Offer") in connection with the Offer by S&T Bancorp, Inc., a Pennsylvania
corporation (the "Company"), to purchase up to 1,500,000 shares of its Common
Stock, $2.50 par value per share (the "Shares"), at prices not less than
$46.00 nor in excess of $49.00 per Share, as specified by tendering
shareholders, upon the terms and subject to the conditions set forth in the
Offer.
 
  The Company will determine the single per Share price, not less than $46.00
nor in excess of $49.00 per Share, net to the seller in cash (the "Purchase
Price"), that it will pay for Shares validly tendered pursuant to the Offer,
taking into account the number of Shares so tendered and the prices specified
by tendering shareholders. The Company will select the Purchase Price that
will allow it to buy 1,500,000 Shares (or such lesser number of Shares as are
validly tendered at prices not less than $46.00 nor in excess of $49.00 per
Share). All Shares properly tendered at prices at or below the Purchase Price
and not withdrawn on or prior to the Expiration Date (as defined in Section 1
of the Offer to Purchase) will be purchased at the Purchase Price, subject to
the terms and conditions of the Offer, including the proration provisions. See
Section 1 of the Offer to Purchase.
 
  Upon the terms and subject to the conditions of the Offer, if, at the
expiration of the Offer, more than 1,500,000 Shares are validly tendered at or
below the Purchase Price and not withdrawn, the Company will buy Shares (i)
from shareholders who owned beneficially as of the close of business on
January 15, 1998, and continue to own beneficially as of the Expiration Date
an aggregate of fewer than 100 Shares who properly tender all their Shares at
prices at or below the Purchase Price, and (ii) then, on a pro rata basis,
from all other shareholders who properly tender at or below the Purchase Price
(and do not withdraw them prior to the expiration of the Offer). See Sections
1 and 2 of the Offer to Purchase. All Shares not purchased pursuant to the
Offer, including Shares tendered at prices greater than the Purchase Price and
Shares not purchased because of proration will be returned to the tendering
shareholders at the Company's expense as promptly as practicable following the
Expiration Date.
 
  We are the owner of record of Shares held for your account. As such, we are
the only ones who can tender your Shares, and then only pursuant to your
instructions. We are sending you the Letter of Transmittal for your
information only; you cannot use it to tender Shares we hold for your account.
 
  Please instruct us as to whether you wish us to tender any or all of the
Shares we hold for your account on the terms and subject to the conditions of
the Offer.
 
  We call your attention to the following:
 
  1. You may tender all or a portion of your Shares at prices not less than
     $46.00 nor in excess of $49.00 per Share as indicated in the attached
     Instruction Form, net to you in cash.
 
  2. The Offer is not conditioned on any minimum number of Shares being
     tendered pursuant to the Offer.
 
Client Letter
<PAGE>
 
  3. The Offer, proration period and withdrawal rights will expire at 5:00
     p.m., Eastern time, on February 18, 1998, unless the Company extends the
     Offer.
 
  4. The Offer is for 1,500,000 Shares, constituting approximately 11% of the
     Shares outstanding as of January 15, 1998.
 
  5. Tendering shareholders will not be obligated to pay any stock transfer
     taxes on the Company's purchase of Shares pursuant to the Offer, subject
     to Instruction 7 of the Letter of Transmittal.
 
  6. If you beneficially held, as of the close of business on January 15,
     1998, an aggregate of fewer than 100 Shares and you continue to
     beneficially own as of the Expiration Date an aggregate of fewer than
     100 Shares, and you instruct us to tender on your behalf all such Shares
     at or below the Purchase Price before the Expiration Date (as defined in
     the Offer to Purchase) and complete the box captioned "Odd Lots" in the
     attached Instruction Form, the Company, upon the terms and subject to
     the conditions of the Offer, will accept all such Shares for purchase
     before proration, if any, of the purchase of other Shares validly
     tendered at or below the Purchase Price.
 
  7. If you wish to tender portions of your Shares at different prices, you
     must complete a separate Instruction Form for each price at which you
     wish to tender each such portion of your Shares. We must submit separate
     Letters of Transmittal on your behalf for each price you will accept.
 
  If you wish to have us tender any or all of your Shares, please so instruct
us by completing, executing, detaching and returning to us the attached
Instruction Form. An envelope to return your Instruction Form to us is
enclosed. If you authorize us to tender your Shares, we will tender all such
Shares unless you specify otherwise on the attached Instruction Form.
 
  YOUR INSTRUCTION FORM SHOULD BE FORWARDED TO US IN AMPLE TIME TO PERMIT US
TO SUBMIT A TENDER ON YOUR BEHALF ON OR BEFORE THE EXPIRATION DATE OF THE
OFFER. THE OFFER, PRORATION PERIOD AND WITHDRAWAL RIGHTS EXPIRE AT 5:00 P.M.,
EASTERN TIME, ON FEBRUARY 18, 1998, UNLESS THE COMPANY EXTENDS THE OFFER.
 
  As described in Section 1 of the Offer to Purchase, if more than 1,500,000
Shares have been validly tendered at prices at or below the Purchase Price and
not withdrawn on or prior to the Expiration Date (as defined in the Offer to
Purchase), the Company will purchase properly tendered Shares on the basis set
forth below:
 
  (a) first, all Shares validly tendered and not withdrawn on or prior to the
      Expiration Date by or on behalf of any shareholder who owned
      beneficially, as of the close of business on January 15, 1998 and
      continues to own beneficially as of the Expiration Date, an aggregate
      of fewer than 100 Shares who:
 
    (1) validly tender all of such Shares at a price at or below the
        Purchase Price (partial tenders will not qualify for this
        preference); and
 
    (2) completes the box captioned "Odd Lots" on the Letter of
        Transmittal; and
 
  (b) second, after purchase of all of the forgoing Shares, and all other
      Shares validly and tendered at or below the Purchase Price and not
      withdrawn on or prior to the Expiration Date on a pro rata basis (with
      appropriate adjustments to avoid purchases of fractional Shares) as
      described in Section 1 of the Offer to Purchase.
 
  The Offer is being made to all holders of Shares. The Company is not aware
of any jurisdiction where the making of the Offer is not in compliance with
applicable law. If the Company becomes aware of any jurisdiction where the
making of the Offer is not in compliance with any valid applicable law, the
Company will make a good faith effort to comply with such law. If, after such
good faith effort, the Company cannot comply with such law, the Offer will not
be made to, nor will tenders be accepted from or on behalf of, holders of
Shares residing in such jurisdiction. In any jurisdiction the securities or
blue sky laws of which require the Offer to be made by a licensed broker or
dealer, the Offer is being made on the Company's behalf by the Dealer Manager
or one or more registered brokers or dealers licensed under the laws of such
jurisdiction.
<PAGE>
 
                               INSTRUCTION FORM
            FOR SHARES HELD BY BROKERS, DEALERS, COMMERCIAL BANKS,
                      TRUST COMPANIES AND OTHER NOMINEES.
 
                       INSTRUCTIONS FOR TENDER OF SHARES
                             OF S&T BANCORP, INC.
 
  Please tender to S&T Bancorp, Inc. (the "Company"), on (our) (my) behalf,
the number of Shares indicated below, which are beneficially owned by (us)
(me) and registered in your name, upon terms and subject to the conditions
contained in the Offer to Purchase of the Company dated January 20, 1998, and
the related Letter of Transmittal, the receipt of both of which is
acknowledged.
 
  The undersigned hereby instruct(s) you to tender to the Company the number
of Shares indicated below, at the price per Share indicated below, pursuant to
the terms and subject to the conditions of the Offer.
 
  Aggregate number of Shares to be tendered by you for us:          Shares
 
  By checking one of the boxes below, the undersigned understands that none of
my Shares will be purchased if the Purchase Price is less than the price
checked.
 
  Price (in dollars) per Share at which Shares are being tendered (See
Instruction 5 on the Letter of Transmittal):
 
                           [_] $46.00   [_]  $48.00
 
                           [_] $46.50   [_]  $48.50
 
                           [_] $47.00   [_]  $49.00
 
                           [_] $47.50
 
                                   ODD LOTS
               (SEE INSTRUCTION 9 ON THE LETTER OF TRANSMITTAL)
 
[_]Check here ONLY if I was the beneficial owner as of the close of business
   on January 15, 1998, and continue to be the beneficial owner as of the
   Expiration Date, of an aggregate of fewer than 100 Shares, all of which are
   being tendered.
 
  THE METHOD OF DELIVERY OF THIS DOCUMENT IS AT THE OPTION AND RISK OF THE
TENDERING SHAREHOLDER. IF DELIVERY IS BY MAIL, REGISTERED MAIL WITH RETURN
RECEIPT REQUESTED, PROPERLY INSURED, IS RECOMMENDED. IN ALL CASES, SUFFICIENT
TIME SHOULD BE ALLOWED TO ASSURE DELIVERY.
 
  THE BOARD OF DIRECTORS OF THE COMPANY HAS UNANIMOUSLY APPROVED THE OFFER.
NEITHER THE COMPANY NOR ITS BOARD OF DIRECTORS, HOWEVER, MAKES ANY
RECOMMENDATION TO ANY SHAREHOLDER AS TO WHETHER TO TENDER ALL OR ANY SHARES.
EACH SHAREHOLDER MUST MAKE HIS OR HER OWN DECISION AS TO WHETHER TO TENDER
SHARES AND, IF SO, HOW MANY TO TENDER AND AT WHAT PRICE. EMPLOYEES, OFFICERS
AND DIRECTORS OF THE COMPANY MAY PARTICIPATE IN THE OFFER ON THE SAME BASIS AS
THE COMPANY'S OTHER SHAREHOLDERS.
<PAGE>
 
Signature(s): _______________________     Address: ____________________________
 
_____________________________________     _____________________________________
                                                       (Including Zip Code)
 
Name(s): _________________________________________
                        (Please Print)
 
 _________________________________________________
                        (Please Print)
 
Area Code and Telephone Number: __________________
 
Date: ______________________________________, 1998
 
__________________________________________________
   (Employer Identification or Social Security
                     Number)
 
  IMPORTANT: SHAREHOLDERS ARE ENCOURAGED TO RETURN A COMPLETED FORM W-9 WITH
THEIR INSTRUCTION FORM.
<PAGE>
 
                                                                 Exhibit (a)(6)
                               S&T BANCORP, INC.
 
January 20, 1998
 
Dear Shareholders:
 
  Over time, the profitable operations of S&T Bancorp have contributed to the
growth of a capital base that exceeds all applicable regulatory standards and
the amount of capital needed to support the Company's banking business. After
evaluating a variety of alternatives to utilize this strong capital base more
effectively and to maximize value to our shareholders, we have determined that
a repurchase of our own shares is currently the best alternative to accomplish
those objectives. The Board of Directors has approved a repurchase of
1,500,000 shares of the Company's common stock, or approximately 11 percent of
our 14,143,655 outstanding shares. A copy of the Offer to Purchase is
enclosed.
 
  The Company is conducting the offer through a procedure referred to as a
"Modified Dutch Auction." This procedure allows you to select the price at
which you are willing to sell, or tender, all or part of your shares within a
price range of not less than $46.00 per share nor in excess of $49.00 per
share. Upon expiration of the offer, we will select the purchase price from
those shares tendered that will allow us to buy 1,500,000 shares. All shares
purchased in the offer will receive the same purchase price, even those shares
that are tendered below the purchase price. In addition, if you own less than
100 shares and tender all of your shares at or below the purchase price, you
will receive priority and have all of your shares purchased even if more than
1,500,000 shares are tendered.
 
  We encourage each shareholder to read carefully the Offer to Purchase and
related materials. Neither S&T Bancorp, Inc. nor our Board of Directors makes
any recommendation whether to tender shares to the Company. You should make
your decision independently after consulting with your advisors.
 
  To assist us with this offer, we have engaged Corporate Investor
Communications, Inc. to serve as Information Agent. Representatives from this
firm may contact you by phone to make sure you have received the Offer to
Purchase and related materials and to answer any questions you may have. If
you need information or additional forms, please call toll-free the
Information Agent at (800) 201-9605.
 
  Unless otherwise extended, the offer will expire at 5:00 p.m. Eastern time
on February 18, 1998. We again encourage you to read carefully the enclosed
material.
 
  As always, we appreciate your interest in S&T Bancorp, Inc.
 
                                          Sincerely,
 
                                          /s/ James C. Miller
                                          -------------------------------------
                                          James C. Miller
                                          President and Chief Executive
                                          Officer
<PAGE>
 
                                                                 Exhibit (a)(7)
                               S&T BANCORP, INC.
 
                         IMMEDIATE ATTENTION REQUIRED
 
                                                               January 20, 1998
 
                   RE: DIRECTION CONCERNING TENDER OF SHARES
 
To: Participants in the Thrift Plan of S&T Bancorp, Inc.
 
  Enclosed are materials that require your immediate attention. They describe
matters directly affecting your interest in the Thrift Plan ("Thrift Plan")
maintained by S&T Bancorp, Inc. (the "Company"). Read all the materials
carefully. You will need to complete the enclosed Direction Form and return it
to the Investment Management and Trust Services Department in the envelope
provided. THE DEADLINE FOR RECEIPT OF YOUR COMPLETED DIRECTION FORM IS 5:00
P.M., EASTERN TIME, ON FEBRUARY 18, 1998 (UNLESS EXTENDED).
 
  The remainder of this letter summarizes the transaction and your rights and
options under the Thrift Plan, but you also should review the more detailed
explanation provided in the other materials.
 
BACKGROUND
 
  The Company has made a tender offer to purchase up to 1,500,000 shares of
its Common Stock (the "Offer"). The objectives of the purchase, and financial
and other information relating to the Offer, are described in detail in the
enclosed Offer to Purchase, which is being provided to all shareholders of the
Company.
 
  As a participant who has shares of common stock of the Company allocated to
your accounts under the Thrift Plan, you are affected, because the Company's
Offer to Purchase extends to the approximately 521,715 shares of the Company's
Common Stock currently held by the Thrift Plan and allocated to participants'
accounts. Only the S&T Bank Investment Management and Trust Services
Department (the "Trustees"), as Trustees of the Thrift Plan, actually can
tender these shares for sale. However, as a participant, you have the right to
direct the Trustees whether to tender your shares of Company Common Stock
allocated to your accounts as of January 15, 1998. If you elect to have the
Trustees tender these shares, you also are entitled to specify the number of
shares and price or prices at which they should be tendered.
 
  To ensure the confidentiality of your decision, the Trustees will tabulate
the directions of all participants in the Thrift Plan. Your Direction Forms
are to be returned to the Trustees. You should note that the Trustees will
determine whether the implementation of any participant directions or
adherence to any Thrift Plan provisions would be a violation of the Employee
Retirement Income Security Act of 1974 ("ERISA"). Although it is not
anticipated that any direction will violate ERISA, such that the direction
would have to be reversed or ignored, the Department of Labor requires that
the Trustees, as the fiduciaries for participants, retain this discretion.
 
HOW THE OFFER TO PURCHASE WORKS
 
  The details of the Offer to Purchase are described in the enclosed
materials, which you should review carefully. However, in broad outline, the
transaction will work as follows with respect to Thrift Plan participants.
 
  .  The Company has offered to purchase up to 1,500,000 of its shares of
     Common Stock at a price between $46.00 and $49.00 per share.
 
  .  If you want any of the shares that are allocated to your account sold,
     you need to direct that they be offered (or "tendered") for sale.
 
Thrift Letter
<PAGE>
 
  .  You may specify the price at which you want the shares tendered, which
     must be between the two limits above.
 
  .  After the deadline for the tender of shares by all shareholders, the
     Depositary for the Offer, will tabulate all directions, and the Company
     will determine the price, between the two limits, at which it can
     purchase up to 1,500,000 shares. This is referred to as the Purchase
     Price.
 
  .  Unless the Offer is voided or discontinued in accordance with its terms,
     the Company then will buy all the shares that were tendered at the
     Purchase Price or below. However, all sellers will receive the same
     Purchase Price, even if they tendered below the Purchase Price.
 
  .  If you have specified a price in excess of the Purchase Price as finally
     determined, those shares will not be purchased, and they will remain
     allocated to your account.
 
  If you do not want to sell any shares allocated to your account, you do not
need to take any action with respect to this Offer. Shares of Common Stock
allocated to your account will remain in your account.
 
  The Trustees may override any direction that it determines is in violation
of ERISA, as previously described. In particular, the Company will be
prohibited from purchasing shares from the Thrift Plan if the Purchase Price,
as finally determined, is less than the prevailing market price of the shares
on the date the shares are accepted for purchase.
 
  Finally, the Company will prorate the number of shares purchased from
shareholders if there is an excess of shares over the exact number desired at
the Purchase Price as ultimately determined.
 
PROCEDURE FOR DIRECTING TRUSTEES
 
  A Direction Form for giving your instructions to the Trustees is enclosed
for each participant in the Thrift Plan. You must complete this form and
return it in the included envelope in time to be received no later than 5:00
p.m., Eastern time, on February 18, 1998 (unless the Offer to Purchase is
extended or amended). If your form is not received by this deadline, or if it
is not fully and properly completed, the shares in your account will not be
tendered.
 
  To properly complete your Direction Form, you must do the following:
 
  (1) On the face of the form, specify the number of shares that you want to
      tender at each price indicated. Typically, you would elect to have all
      of your shares tendered at a single price. However, the form gives you
      the option of splitting your shares among several prices. You must
      state the number of shares to be sold at each indicated price by
      filling in the number of shares in the box immediately below the price.
 
  (2) Date and sign the Direction Form in the spaces provided.
 
  (3) Return the Direction Form in the included envelope to the Trustees no
      later than 5:00 p.m., Eastern Time, on February 18, 1998 (unless this
      deadline is extended).
 
Your direction will be deemed irrevocable unless withdrawn by 5:00 p.m.,
Eastern time, on February 18, 1998 (unless this deadline is extended). To be
effective, you must contact the Trustees and instruct the Trustees to withdraw
such tender of shares. Only the Trustees can actually withdraw your tender of
shares.
 
  Upon receipt of your notice, your previous direction will be deemed
cancelled. You may direct the retendering of any shares in your account by
repeating the procedures set forth in this letter.
<PAGE>
 
INVESTMENT OF TENDER PROCEEDS
 
  INDIVIDUAL PARTICIPANTS IN THE THRIFT PLAN WILL NOT RECEIVE ANY PORTION OF
THE TENDER PROCEEDS. ALL SUCH PROCEEDS AND THE ASSETS WILL REMAIN IN THE PLAN
AND MAY BE WITHDRAWN ONLY IN ACCORDANCE WITH THE TERMS OF THE PLAN. NO GAIN OR
LOSS WILL BE RECOGNIZED BY PARTICIPANTS IN THE THRIFT PLAN FOR FEDERAL INCOME
TAX PURPOSES IN CONNECTION WITH THE TENDER OR SALE OF SHARES HELD IN THE PLAN.
 
NO RECOMMENDATION
 
  THE COMPANY'S BOARD OF DIRECTORS HAS UNANIMOUSLY APPROVED THE MAKING OF THE
OFFER TO PURCHASE. HOWEVER, NEITHER THE COMPANY, ITS BOARD OF DIRECTORS, THE
TRUSTEES, THE DEALER MANAGER, THE INFORMATION AGENT, OR ANY OTHER PERSON MAKES
ANY RECOMMENDATION TO PARTICIPANTS AS TO WHETHER TO TENDER SHARES, THE PRICE
AT WHICH TO TENDER, OR WHETHER TO REFRAIN FROM TENDERING SHARES. EACH
PARTICIPANT MUST MAKE HIS OR HER OWN DECISION ON THESE MATTERS.
 
CONFIDENTIALITY
 
  YOUR DECISION WILL NOT BE DISCLOSED TO ANY DIRECTOR, OFFICER, OR EMPLOYEE OF
S&T BANCORP, INC., EXCEPT FOR THE PURPOSE OF ALLOCATING PROCEEDS TO YOUR
ACCOUNT IN THE EVENT THAT ALL OR A PORTION OF YOUR SHARES ARE SOLD.
 
FURTHER INFORMATION
 
  Although Corporate Investor Communications, Inc. (the "Information Agent")
has no recommendation and cannot advise you what to do, its representatives
are prepared to answer any question that you may have on the procedures
involved in the Offer and your direction. The Information Agent also can help
you complete your Direction Form.
 
  For this purpose, you may contact the Information Agent at the following
number between 9:00 a.m. and 5:00 p.m., Eastern time, Monday through Friday:
 
                       CORPORATE INVESTOR COMMUNICATIONS
                               111 Commerce Road
                              Carlstadt, NJ 07072
 
                           Toll Free (800) 201-9605
 
  Your ability to instruct the Trustees concerning whether to tender shares
allocated your account is an important part of your rights as a Thrift Plan
participant. Please consider this letter and the enclosed materials carefully
and then return your Direction Form promptly if you desire to tender.
 
                                          Sincerely,
 
                                          /s/ James C. Miller
                                          -------------------------------------
                                          President and Chief Executive
                                          Officer
<PAGE>
  
                               S&T BANCORP, INC.
                                  THRIFT PLAN
                                 DIRECTION FORM
  BEFORE COMPLETING THIS FORM, PLEASE READ CAREFULLY THE ACCOMPANYING OFFER TO
                                    PURCHASE
 SEE THE ADDRESS LABEL BELOW FOR THE NUMBER OF SHARES ALLOCATED TO YOUR ACCOUNT
 
  In accordance with the S&T Bancorp, Inc. (the "Company") Offer to Purchase
dated January 20, 1998, a copy of which I have received and read, and pursuant
to the pass-through provisions of the Company's Thrift Plan (the "Plan"), I
hereby direct the Plan's Trustees
 
  To tender the number of shares allocated to my account at the price or
  prices indicated below:
 
<TABLE>
   <S>                <C>    <C>    <C>    <C>    <C>    <C>    <C>
   PRICE:             $46.00 $46.50 $47.00 $47.50 $48.00 $48.50 $49.00
   NUMBER OF
    SHARES TENDERED:
                      ------ ------ ------ ------ ------ ------ ------
</TABLE>
 
                                  INSTRUCTIONS
 
  Carefully complete the Direction Form above. Then insert today's date and
sign your name in the space provided below. Enclose the form in the included
envelope and return it promptly to the Investment Management and Trust Services
Department. Your Direction Form must be received no later than 5:00 p.m.,
Eastern time, on February 18, 1998. Direction Forms that are not fully or
properly completed, dated, and signed, or that are received after the deadline,
will be ignored, and the shares allocated to your account will not be tendered.
Note that the Trustees also have the right to ignore any direction that it
determines cannot be implemented without violation of ERISA.
 
  Neither the Company, its Board of Directors, the Trustees, nor any other
party makes any recommendation to participants as to whether to tender shares,
the price at which to tender, or to refrain from tendering shares. Each
participant must make his or her own decision on these matters.
 
  As of January 15, 1998, there were allocated to your account the number of
shares of common stock on the label below.
 
                                              Date:          , 1998


                                              ----------------------------------
                                                        Your Signature
                                              (Please sign as your name appears
                                                            below)
<PAGE>
 
                                                                 Exhibit (a)(8)
                                  MEMORANDUM
 
TO:ALL STAFF
 
FROM:JIM MILLER
 
DATE:JANUARY 20, 1998
 
RE:TENDER OFFER FOR THE COMMON STOCK OF S&T BANCORP, INC.
 
  The Board of Directors of S&T Bancorp, Inc. has approved the purchase of
1,500,000 shares of our common stock by means of what is termed a "Modified
Dutch Auction Tender." We have made every effort to communicate this action to
you as quickly as possible. Below you will find the answers to some questions
that are likely to arise from our public announcement, which was made this
morning. We will provide further information if additional questions come up.
 
QUESTION: Why is S&T Bancorp, Inc. offering to repurchase its stock?
 
ANSWER: Over time, S&T's profitable operations have contributed to the growth
        of a capital base that exceeds all regulatory standards and the amount
        of capital to support our banking business. After evaluating a variety
        of alternatives to utilize this strong capital base more effectively
        and to attempt to maximize shareholder value, management and its Board
        of Directors determined that a purchase of our shares is currently the
        best alternative to accomplish these objectives.
 
QUESTION: What is a "Modified Dutch Auction"?
 
ANSWER: A Modified Dutch Auction is a process whereby a company makes a direct
        tender offer to its own shareholders to purchase a specified number of
        shares of its stock within a specified price range per share, and pays
        the highest price at which it accepts shares to all shareholders whose
        shares are accepted. In this case, the Company is making a direct
        offer to all of its shareholders to purchase in the aggregate
        1,500,000 shares of common stock at a price not less than $46.00 nor
        in excess of $49.00 per share. This process allows each shareholder to
        elect whether to sell stock, and the price the shareholder is willing
        to sell at within the given price range. After receiving tenders of
        shares, at the termination of the Offer, the Company will choose the
        price within the specified range that will permit it to purchase the
        amount of securities sought, and this price will become the Purchase
        Price.
 
QUESTION: How should I respond to questions?
 
ANSWER: S&T Bancorp, Inc. has hired a special Information Agent to handle all
        questions. The Information Agent is Corporate Investor Communications,
        Inc., and their toll-free telephone number is (800) 201-9605. Because
        S&T Bancorp, Inc. is the purchaser of the shares, and because
        securities laws are involved, it is very important that all questions
        be referred to the Information Agent. No staff member of S&T Bancorp,
        Inc. is allowed to answer any questions or give any advice regarding
        the tender offer. We are aware that many shareholders are customers of
        the bank and have ties or relationships with staff members. You should
        handle these situations as diplomatically as possible, but in any
        event, all questions must be referred either to the Information Agent
        or the holder's broker or investment advisor.
 
QUESTION: What do I say if a shareholder asks, "Should I sell (tender) my
          stock?"
 
ANSWER: Members of the S&T Bancorp, Inc. staff must not give any investment
        advice to shareholders. The shareholder must make his or her own
        investment decision. You should not express an opinion as to whether
        you think the tender offer is a "good deal" or a "bad deal." While the
        shareholder may call the Information Agent or the Dealer Manager, they
        will not receive investment advice from either organization. They
        should be directed to contact their broker or investment advisor.
 
QUESTION: What do I do if someone brings a Letter of Transmittal to me or my
          office or sends it to our office by mail?

  
<PAGE>
 
ANSWER: Because tenders must be received by ChaseMellon Shareholder Services,
        L.L.C., within a limited amount of time, we cannot take the
        responsibility for having any shareholder's tender delivered by mail
        or by hand. Shareholders must send tenders directly to ChaseMellon
        Shareholder Services, L.L.C. at the address provided in the tender
        offer documents. That address is:
 
                   CHASEMELLON SHAREHOLDER SERVICES, L.L.C.
 
               By Mail:
 
                                                 Overnight Delivery:
 
             P.O. Box 3301                       85 Challenger Road
      South Hackensack, NJ 07606                   Mail Drop Reorg    
                                              Ridgefield Park, NJ 07660
                                              Attn: Reorganization Dept. 
                                             
 
                                   By Hand:
 
                                 120 Broadway
                                  13th Floor
                              New York, NY 10271
 
QUESTION: May employees of S&T Bancorp, Inc. tender shares in the offer?
 
ANSWER: Yes. Employees who own shares of S&T Bancorp, Inc. common stock are
        eligible to tender their shares. You will receive a complete copy of
        the same documents that are being provided to other shareholders.
 
QUESTION: May employees tender shares they own in the Thrift Plan?
 
ANSWER: Yes. We are enclosing a complete copy of all of the documents from the
        Investment Management and Trust Services Department who are trustees
        for the plans. Just as you vote your shares through the proxy process
        at the annual meeting, you have to instruct the trustees (Investment
        Management and Trust Services) if you want to tender some or all of
        your 401k/ESOP shares. Remember, if you do tender, to provide Employee
        Services with a subsequent form of how to reallocate your investments,
        i.e., direction on how you want this cash reinvested. Your decision
        will be kept confidential.
<PAGE>
 
                                                                  Exhibit (a)(9)
 
                               S&T BANCORP, INC.
 
                             QUESTIONS AND ANSWERS
                               ABOUT THE OFFER OF
                               S&T BANCORP, INC.
 
                  TO PURCHASE FOR CASH UP TO 1,500,000 SHARES
                     OF COMMON STOCK AT A PURCHASE PRICE OF
                           $46.00 TO $49.00 PER SHARE
 
 
 
                                JANUARY 20, 1998
<PAGE>
 
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
 
                   QUESTIONS AND ANSWERS ABOUT THE OFFER OF
                               S&T BANCORP, INC.
                             TO PURCHASE ITS STOCK
 
  The following information is designed to answer frequently asked questions
about the offer by S&T Bancorp, Inc. (the "Company") to purchase shares of its
common stock. Shareholders are referred to the Offer to Purchase and Letter of
Transmittal for a detailed description of the terms and conditions of the of-
fer.
 
Q. WHY IS THE COMPANY MAKING THIS OFFER?
A.The Company believes it has a strong and more than adequate capital base
which will allow them to continue to grow their business and using the addi-
tional capital to buyback stock will allow them to increase shareholder value.
The repurchasing of stock is designed to increase the Company's return on eq-
uity by reducing the amount of equity outstanding.
 
Q. WHAT IS THIS OFFER TO PURCHASE?
A.The Company is inviting its shareholders to tender shares of its common
stock at prices not less than $46.00 nor in excess of $49.00 per Share in
cash, as specified by shareholders tendering their Shares. The Company will
determine the single per Share price, not less than $46.00 nor in excess of
$49.00 per Share, net to the seller in cash (the "Purchase Price"), that it
will pay for Shares validly tendered pursuant to the Offer, taking into ac-
count the number of Shares so tendered and the prices specified by tendering
shareholders. The Company will select the Purchase Price that will allow it to
buy 1,500,000 Shares (or such lesser number of Shares as are properly tendered
at prices not less than $46.00 nor in excess of $49.00 per Share). This type
of issuer tender offer is commonly referred to as a "Modified Dutch Auction."
 
Q. WHAT IS A "MODIFIED DUTCH AUCTION?"
A.A Modified Dutch Auction is a process whereby a company makes a direct ten-
der offer to its own shareholders to purchase a specified number of shares of
its stock within a specified price range per share, and pays the highest price
at which it accepts shares to all shareholders whose shares are accepted. In
this case, the Company is making a direct offer to all of its shareholders to
purchase in the aggregate 1,500,000 Shares of its common stock at a price not
less than $46.00 nor in excess of $49.00 per Share. This process allows each
shareholder to elect whether to sell stock, and the price the shareholder is
willing to sell at within the given price range. After receiving tenders of
Shares, at the termination of the Offer, the Company will choose the price
within the specified range that will permit it to purchase the amount of secu-
rities sought and this price will become the Purchase Price.
 
Q. WHAT WILL BE THE FINAL PURCHASE PRICE?
A.All Shares acquired in the Offer will be acquired at the Purchase Price. The
Company will select the Purchase Price that will allow it to buy up to
1,500,000 Shares. All shareholders tendering at or below the Purchase Price
will receive the same amount.
 
Q.WHAT WILL HAPPEN IF MORE THAN 1,500,000 SHARES ARE TENDERED AT OR BELOW THE
PURCHASE PRICE?
A.In the event more than 1,500,000 Shares are tendered at or below the Pur-
chase Price, Shares tendered at or below the Purchase Price will be acquired
by the Company (i) first from any shareholder who owned beneficially, as of
the close of business on January 15, 1998 and continues to own beneficially as
of the termination of the Offer, an aggregate of fewer than 100 Shares and who
validly tenders all of such Shares, and (ii) then from all other tendering
shareholders subject to proration.
 
Q. AT WHAT PRICE MAY I TENDER MY SHARES?
A.Shareholders may elect to tender their Shares in increments of 1/2 of a dol-
lar ($.50) starting at $46.00 per Share up to and including $49.00 per Share.
The election as to the number of Shares and the price a shareholder is willing
to tender are to be indicated on the Letter of Transmittal.
 
Q. HOW DO I TENDER MY SHARES?
A.If you hold your Shares in certificate form, you must return a properly com-
pleted Letter of Transmittal (the blue form) and any other documents required
by the Letter of Transmittal, together with the certificates for the Shares
being tendered, to the Depositary, ChaseMellon Shareholder Services, L.L.C.,
which must be received by them by 5:00 p.m. Eastern time on February 18, 1998.
  
PLEASE DO NOT ENDORSE YOUR CERTIFICATE(S).
<PAGE>
 
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
 
Q. HOW DO I TENDER MY SHARES IF MY SHARES ARE HELD BY MY BROKER?
A. If your Shares are registered in street name with a broker, dealer, commer-
cial bank, trust company or other nominee, you will need to contact your bro-
ker, bank or other nominee and instruct the nominee to make the tender of your
Shares for you. You cannot tender such Shares using the Letter of Transmittal
even though you may have received one for your information.
 
If you are a broker and are tendering Shares in book-entry form for your cus-
tomers, you must comply with the Book-Entry Delivery procedure described in
Section 3 of the Offer to Purchase.
 
Q. WHAT DO I DO IF I HAVE LOST MY CERTIFICATES, OR IF THEY HAVE BEEN MUTILATED,
DESTROYED OR STOLEN, BUT I STILL WANT TO TENDER THEM?
A. Call the Depositary at (888) 884-8091 for instructions for tendering Shares
in such circumstances.
 
Q. I WANT TO TENDER BUT I CANNOT GET MY STOCK TO THE DEPOSITARY ON TIME. WHAT
CAN I DO?
A. If you cannot submit a valid tender by the expiration date but want to ten-
der, you may complete the guaranteed delivery instructions which gives you
three days to produce the certificates. Have an eligible institution help you
fill out the form as instructed in Section 3 of the Offer to Purchase.
 
Q. DO I HAVE TO SELL MY STOCK TO THE COMPANY?
A. No. A shareholder is not required to tender any stock.
 
Q. WHAT HAPPENS IF I DO NOT TENDER MY STOCK TO THE COMPANY TO PURCHASE?
A. Nothing will happen if you do not tender any or all of your Shares. Your
Shares will remain outstanding without a change in the terms or ownership
rights. You will continue to own the same number of Shares without any adjust-
ment, and you will continue to receive the same dividend and voting rights.
However, since the Company will purchase up to 1,500,000 of its outstanding
Shares, the percentage of the outstanding stock which you own will increase
since the number of outstanding Shares will be reduced.
 
Q. IF I DO TENDER MY SHARES, WHEN WILL I RECEIVE THE MONEY?
A. As soon as practicable after the Expiration Date. If you are a registered
shareholder, you will receive a check from the Depositary or if you hold your
stock with a bank or broker your account will be credited.
 
Q. WHAT IF THE TERMS OF THE OFFER CHANGE?
A. In the event the Expiration Date is extended or if the terms of the Offer
are materially changed, the Company will generally give notice of the change
and, under certain circumstances, shareholders will be able to change or with-
draw their tender for at least 10 business days from such notice.
 
Q. CAN I TENDER PART OF MY STOCK AT DIFFERENT PRICES?
A. Yes, you can elect to tender part of your stock at one price and an addi-
tional amount at a second price. For example, if you owned 1,500 Shares, you
could tender 500 Shares at one price, 500 Shares at another and keep the re-
maining 500 Shares. However, you cannot tender the same stock at different
prices. In the prior example, the shareholder owning 1,500 Shares cannot ten-
der 1,500 at one price and 1,500 at another price. If you tender some Shares
at one price and other Shares at a different price, you must use a separate
Letter of Transmittal for each price.
 
Q. IS THERE ANY BROKERAGE COMMISSION?
A. No. The Company will purchase stock directly from each shareholder at the
Purchase Price without the use of a broker.
 
Q. CAN I CHANGE OR CANCEL MY TENDER?
A. You may increase or decrease the number of Shares indicated in the Letter of
Transmittal or withdraw it entirely up until 5:00p.m. Eastern time on February
18, 1998. Generally after February 18, 1998, you cannot withdraw your tender.
If you desire to change or withdraw your tender, you are responsible to make
certain that a valid withdrawal is received by the February 18, 1998 deadline.
Except as discussed in the Offer to Purchase, tenders are irrevocable after
the February 18, 1998 deadline.
<PAGE>
 
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
Q. CAN YOU SUMMARIZE THE PROCESS BY WHICH SHARES ARE VALIDLY TENDERED?
A. Generally, for certificated Shares you must complete the Letter of Transmit-
tal (the blue form) as follows:
 
 . List the certificates and the number of Shares that you are tendering in
   the box captioned "Description of Shares Tendered".
 . Check the box specifying the price at which you are tendering in the box
   captioned "Price (in Dollars) Per Share at Which Shares are Being Ten-
   dered".
 . If you want to give us special payment instructions, complete the box cap-
   tioned "Special Payment Instructions".
 . If you want to give us special delivery instructions, complete the box cap-
   tioned "Special Delivery Instructions".
 . If you are an Odd Lot Holder (i.e., you hold fewer than 100 Shares) who is
   tendering all your shares, complete the box captioned "Odd Lots".
 . If your Shares are being delivered by book-entry, complete the box cap-
   tioned "Box Below for Use By Eligible Institutions Only".
 . Complete the substitute Form W-9 to certify your tax identification number.
 . Sign the Letter of Transmittal in the box captioned "Sign Here" (in certain
   circumstances, signatures must be guaranteed in this Box, see Instructions
   1 and 6 in the Letter of Transmittal).
 
You must deliver your Share certificates or comply with the book-entry deliv-
ery requirements. See Section 3 of the Offer to Purchase. These documents must
be received by the Depositary, ChaseMellon Shareholder Services, L.L.C., no
later than 5:00 p.m. Eastern time on February 18, 1998. If you are tendering
Shares held by a broker, commercial bank, trust company or other nominee, your
instructions must be given to your nominee who will, on the basis of your in-
structions, tender Shares for you. Please see Section 3 and the Letter of
Transmittal for more details about how to tender Shares.
 
Q. CAN I TENDER SHARES HELD IN THE COMPANY'S DIVIDEND REINVESTMENT PLAN?
A. Yes, shares credited to your account will be tendered by the Depositary ac-
cording to instructions provided by you.
 
Q. HOW CAN I GET MORE INFORMATION?
A. If you have a question, please call our Information Agent, Corporate In-
vestor Communications, Inc., at (800) 201-9605 from 9:00 a.m.-5:00 p.m., East-
ern time Monday through Friday.
 
 
 
THIS BROCHURE IS NEITHER AN OFFER TO PURCHASE NOR A SOLICITATION OF AN OFFER
TO SELL SECURITIES. THE OFFER TO PURCHASE THE STOCK OF THE COMPANY IS MADE
ONLY BY THE S&T BANCORP, INC. OFFER TO PURCHASE DOCUMENT DATED JANUARY 20,
1998 AND THE ACCOMPANYING LETTER OF TRANSMITTAL.
<PAGE>
 
                                                                Exhibit (a)(10)
 
FOR IMMEDIATE RELEASE
Date: January 20, 1998
 
Contact: Jim Barone, Treasurer
412-465-1417
 
                               S&T BANCORP, INC.
 
                                   ANNOUNCES
            OFFER TO BUY UP TO 1,500,000 SHARES OF ITS COMMON STOCK
 
  INDIANA, PENNSYLVANIA (Nasdaq NMS: STBA)--S&T Bancorp, Inc. announced today
that its Board of Directors has authorized the repurchase of up to 1,500,000
shares of its common stock, which represents approximately 11 percent of its
outstanding shares as of January 15, 1998. The repurchase will be made through
a "Modified Dutch Auction Tender." Under this procedure, S&T Bancorp
shareholders will be given the opportunity to sell part or all of their shares
to the Corporation at a price of not less than $46.00 per share and not more
than $49.00 per share. The offer to purchase shares is commencing immediately
and will expire at 5:00 p.m. Eastern time on February 18, 1998 unless extended
by S&T Bancorp, Inc.
 
  Under the procedures for a Modified Dutch Auction Tender, shareholders may
offer to sell all or a portion of the shares they own within a price range of
not less than the minimum price of $46.00 and not in excess of the maximum
price of $49.00 specified in the tender. Upon the expiration of the offer, S&T
Bancorp, Inc. will select the purchase price that will allow it to buy
1,500,000 shares. All shares purchased in the offer will receive the same
price. If the number of shares tendered is equal to or less than 1,500,000
shares, the purchase price will be the highest price specified by tendering
shareholders. If the number of shares tendered is greater than the number
sought, the Corporation will select the lowest price that will allow it to buy
the number of shares it seeks.
 
  James C. Miller, President and Chief Executive Officer of S&T Bancorp, Inc.,
stated, "S&T Bancorp, Inc. is announcing this tender offer because the Board
of Directors believes that the purchase of shares pursuant to the offer should
have beneficial effects on shareholder value while maintaining a strong
capital base to support the needs of our business and our customers. After
studying a number of alternatives, we selected the Modified Dutch Auction
Tender, because it is a positive action that has the potential for improving
shareholder returns in an expeditious manner. If the tender offer is
successful, it is expected to improve our return on equity, as well as our
earnings per share."
 
  At December 31, 1997, S&T Bancorp had assets of $1.9 billion and
shareholders' equity of $260 million.
 
  Headquartered in Indiana, PA, S&T Bank, the principal subsidiary of S&T
Bancorp, Inc., operates 38 offices within Armstrong, Allegheny, Indiana,
Jefferson, Clearfield and Westmoreland counties.
 
  This announcement is neither an offer to purchase nor a solicitation of an
offer to sell shares of S&T Bancorp, Inc. common stock. The offer may be made
solely by the Offer to Purchase and the related Letter of Transmittal, which
will be sent to all shareholders upon commencement of the offer.
<PAGE>
 
                                                                Exhibit (a)(11)
 
  This announcement is neither an Offer to Purchase nor a solicitation of an
offer to sell shares of S&T Bancorp, Inc. common stock. The offer is made
solely by the Offer to Purchase, dated January 20, 1998, and the related
Letter of Transmittal, copies of which may be obtained from the Information
Agent.
 
                               S&T BANCORP, INC.
 
                                   OFFERS TO
 
                PURCHASE FOR CASH UP TO 1,500,000 SHARES OF ITS
                                 COMMON STOCK
 
           AT A PURCHASE PRICE NOT LESS THAN $46.00 NOR IN EXCESS OF
 
                               $49.00 PER SHARE
 
          THE OFFER, PRORATION PERIOD AND WITHDRAWAL RIGHTS EXPIRE AT
                5:00 P.M., EASTERN TIME, ON FEBRUARY 18, 1998,
                         UNLESS THE OFFER IS EXTENDED.
 
                    THE INFORMATION AGENT FOR THE OFFER IS:
 
                    CORPORATE INVESTOR COMMUNICATIONS, INC.
                               111 COMMERCE ROAD
                              CARLSTADT, NJ 07072
 
                                (201) 896-1900
 
                     THE DEALER MANAGER FOR THE OFFER IS:
 
                         KEEFE, BRUYETTE & WOODS, INC.
                               TWO WORLD CENTER
                              NEW YORK, NY 10048
 
                                (212) 323-8450
 
January 20, 1998
<PAGE>
 
                                                                Exhibit (a)(12)
 
                                                            CONTACT: JIM BARONE
                                                                   412-465-1417
 
TO BE RELEASED:
Tuesday, January 20, 1998
At 8:15 a.m.
 
                   S&T BANCORP ANNOUNCES INCREASED EARNINGS
 
Indiana, Pennsylvania (Nasdaq NMS:STBA)--S&T Bancorp today reported record
earnings for the year ending December 31, 1997. Fourth quarter net income of
$8,711,000 or $0.62 per share exceeded last year's quarterly results of
$6,935,000 and $0.49 per share by approximately 26 percent. For the year
ending December 31, 1997, net income increased 18 percent to $33,414,000 from
$28,241,000 in 1996. Basic earnings per share increased 18 percent to $2.36 in
1997 from $2.00 per share in 1996. Diluted earnings per share also increased
18 percent to $2.34 from $1.99. All prior period amounts have been restated to
reflect the pooling of interests transaction with Peoples Bank of Unity, which
closed on May 2, 1997.
 
  Return on assets was 1.84 percent for 1997 versus 1.65 percent in 1996.
Return on equity increased to 13.71 percent in 1997 from 13.01 in 1996. James
C. Miller, President and Chief Executive Officer, stated, "We are very pleased
with our 1997 results. Besides completing and converting the Peoples Bank of
Unity acquisition, we were able to show strong revenue growth in both net
interest and noninterest income."
 
  Net interest income increased approximately 7 percent in 1997. Gross loans
increased 6 percent or $74,000,000 and ended the year at $1.274 billion.
Commercial loans provided all of the growth as consumer loans declined due to
the de-emphasis of indirect automobile lending. Miller added, "Commercial
mortgage demand was particularly strong throughout the year, especially in our
newer markets of Allegheny and Westmoreland counties. The referral network we
have built in recent years continues to make us aware of available
transactions, and our responsive personalized service has taken advantage of
those opportunities. In addition, we expanded our securities portfolio by
approximately $70,000,000 in the fourth quarter to take advantage of a
declining interest rate environment, and this had a positive effect on net
interest income."
 
  Noninterest income, excluding security gains, increased approximately 11
percent for the year, led by a 12 percent increase in trust fees and a 14
percent increase in deposit and other service charges.
 
  Security gains and other gains on sales of assets amounted to $5,697,000 in
1997 versus $2,310,000 in 1996. Most of these gains resulted from equity sales
at S&T Investment Company. In the fourth quarter of 1997, S&T Investment
Company donated marketable securities to the S&T Bancorp Charitable
Foundation, which resulted in an approximate $571,000 security gain and a
$795,000 increase in noninterest expense. S&T Investment Company had
unrealized gains of approximately $58,000,000 at December 31, 1997.
 
  Asset quality continues to be very strong. Nonperforming loans were
$3,602,000 at December 31, 1997, down from $10,268,000 on December 31, 1996.
Nonperforming loans represent 0.28 percent of total loans. The allowance for
loan losses was $20,427,000 or 1.60 percent of total loans at December 31,
1997.
 
  S&T Bancorp increased its dividend to $0.30 per share on December 15, 1997.
This was the second increase during 1997 with dividends declared rising 18
percent over the preceding year. In addition, book value per share grew from
$16.02 on December 31, 1996 to $18.39 on December 31, 1997. The market value
of S&T stock increased 41 percent during 1997 and closed the year at $43.25.


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