
<PAGE>
 
                                                              EXHIBIT 99.4(b)(a)

                      AMENDMENT NO. 1 TO CREDIT AGREEMENT


     AMENDMENT dated as of May 7, 1998 to the Credit Agreement dated as of
January 7, 1998 (the "CREDIT AGREEMENT") among ARAMARK SERVICES, INC., ARAMARK
UNIFORM SERVICES GROUP, INC. and ARAMARK CORPORATION, the BANKS party thereto
and THE CHASE MANHATTAN BANK and MORGAN GUARANTY TRUST COMPANY OF NEW YORK, as
Agents.

                              W I T N E S S E T H :

     WHEREAS, the parties hereto desire to amend the Credit Agreement as set
forth below;

     NOW, THEREFORE, the parties hereto agree as follows:

     Section 1.  Defined Terms; References.  Unless otherwise specifically
defined herein, each term used herein which is defined in the Credit Agreement
has the meaning assigned to such term in the Credit Agreement. Each reference to
"hereof", "hereunder", "herein" and "hereby" and each other similar reference
and each reference to "this Agreement" and each other similar reference
contained in the Credit Agreement shall, after this Amendment becomes effective,
refer to the Credit Agreement as amended hereby.

     Section 2.  Definition Amendment.  Section 1.01 of the Credit Agreement is
amended by adding the following definition:

         "TENDER OFFER" means an offer by the Parent Guarantor to buy its shares
     of common stock from the holders thereof at a price and upon terms and
     conditions to be determined by the board of directors of the Parent
     Guarantor or an authorized committee thereof, and the subsequent purchase
     by the Parent Guarantor of such shares, all of which shall be completed no
     later than August 15, 1998.

     Section 3.  Covenant Amendment.  (a)  Section 5.11 of the Credit Agreement
is amended to read in its entirety as follows:

         Section 5.11.  Minimum Consolidated Net Worth  Consolidated Net Worth
     shall (i) at no date on or after October 3, 1997 be less than negative
     $300,000,000, (ii at no date on or after October 2, 1998 be less than
     negative $250,000,000, (iii) at no date on or after October 1, 1999 be 
<PAGE>
 
     less than negative $150,000,000, (iv) at no date on or after September 29,
     2000 be less than negative $50,000,000 and (v) at no date on or after
     September 28, 2001 be less than negative $50,000,000 plus an amount equal
     to 50% of Consolidated Net Income for each Fiscal Year ending on or after
     September 29, 2001 but prior to the date of determination for which
     Consolidated Net Income is positive (but with no deduction on account of
     negative Consolidated Net Income for any fiscal year of the Parent
     Guarantor); provided, however, that the amounts set forth in clause (i)
     through (v) inclusive shall be adjusted upward by $25,000,000 for each full
     multiple of $25,000,000 by which the actual cost of funding the Tender
     Offer is less than $700,000,000. The Parent Guarantor will provide to the
     Agents a certificate of a Principal Officer setting forth the calculation
     of the actual cost of the Tender Offer within ten Domestic Business Days of
     the completion of such Tender Offer.

     (b) The first sentence of Section 5.13 of the Credit Agreement is amended
to read in its entirety as follows:

          The proceeds of Credits hereunder will be used for general corporate
     purposes, including in connection with the Tender Offer or other
     repurchases of shares.

          Section 4.  Representations and Warranties.  The Parent Guarantor and
Borrowers jointly and severally represent and warrant that (i) the
representations and warranties set forth in Article 4 of the Credit Agreement
will be true on and as of the Amendment Effective Date and (ii) no Default will
have occurred and be continuing on such date.

     Section 5.  Governing Law.  This Amendment shall be governed by and
construed in accordance with the laws of the State of New York.

     Section 6.  Counterparts.  This Amendment may be signed in any number of
counterparts, each of which shall be an original, with the same effect as if the
signatures thereto and hereto were upon the same instrument.

     Section 7.  Effectiveness.  This Amendment shall become effective on the
later of (i) the completion of the Tender Offer upon terms and conditions
approved by the board of directors of the Parent Guarantor, or an authorized
committee thereof, and (ii)  the date when the Agents shall have received from
each of the Parent Guarantor, the Borrowers and Banks comprising the Required
Banks a counterpart hereof signed by such party or facsimile or other written
confirmation (in form satisfactory to the Agents) that such party has signed a
counterpart hereof.

                                       2
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IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be duly
executed as of the date first above written.

                         ARAMARK SERVICES, INC.


                         By:_____________________________
                            Title:
                            Address:

 
                         ARAMARK UNIFORM SERVICES GROUP, INC.


                         By:_____________________________
                            Title:
                            Address:

                         ARAMARK CORPORATION


                         By:_____________________________
                            Title:
                            Address:


                         THE CHASE MANHATTAN BANK


                         By:_____________________________
                            Title:
                            Address:
 
                         MORGAN GUARANTY TRUST             
                         COMPANY OF NEW YORK


                         By:_____________________________
                            Title:
                            Address:

                                       3
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                         BANK OF AMERICA NATIONAL    
                         TRUST AND SAVINGS ASSOCIATION


                         By:_____________________________
                            Title:
                            Address:


                         CORESTATES BANK, N.A.


                         By:_____________________________
                            Title:
                            Address:


                         CREDIT LYONNAIS
                            NEW YORK BRANCH


                         By:_____________________________
                            Title:
                            Address:


                         FIRST UNION NATIONAL BANK


                         By:_____________________________
                            Title:
                            Address:


                         NATIONSBANK, N.A.


                         By:_____________________________
                            Title:
                            Address:
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                         PNC BANK NATIONAL ASSOCIATION


                         By:_____________________________
                            Title:
                            Address:


                         THE BANK OF NEW YORK


                         By:_____________________________
                            Title:
                            Address:


                         THE SUMITOMO BANK, LIMITED
                            NEW YORK BRANCH


                         By:_____________________________
                            Title:
                            Address:


                         WACHOVIA BANK, N.A.


                         By:_____________________________
                            Title:
                            Address:


                         BANK OF HAWAII


                         By:_____________________________
                            Title:
                            Address:
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                         THE BANK OF NOVA SCOTIA


                         By:_____________________________
                            Title:
                            Address:


                         BANK ONE, N.A.


                         By:_____________________________
                            Title:
                            Address:


                         BANKBOSTON, N.A.


                         By:_____________________________
                            Title:
                            Address:


                         BHF-BANK AKTIENGESELLSCHAFT


                         By:_____________________________
                            Title:
                            Address:


                         CIBC INC.


                         By:_____________________________
                            Title:
                            Address:
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                         COMERICA BANK


                         By:_____________________________
                            Title:
                            Address:


                         FLEET NATIONAL BANK


                         By:_____________________________
                            Title:
                            Address:


                         KREDIETBANK N.V.


                         By:_____________________________
                            Title:
                            Address:


                         MELLON BANK, N.A.


                         By:_____________________________
                            Title:
                            Address:


                         NATIONAL WESTMINSTER BANK PLC


                         By:_____________________________
                            Title:
                            Address:
<PAGE>
 
                         COOPERATIEVE CENTRALE
                           RAIFFEISEN-BOERENLEENBANK
                           B.A., "RABOBANK NEDERLAND",
                           NEW YORK BRANCH


                         By:_____________________________
                            Title:
                            Address:


                         THE LONG-TERM CREDIT BANK OF
                            JAPAN, LTD., NEW YORK BRANCH


                         By:_____________________________
                            Title:
                            Address:


                         U.S. BANK NATIONAL ASSOCIATION


                         By:_____________________________
                            Title:
                            Address:
