<PAGE>
                                                                  EXECUTION COPY

                                                                   Exhibit 10.40

                    FIRST AMENDMENT dated as of October 12, 2005 (this
               "Amendment"), to the FIVE YEAR CREDIT AGREEMENT dated as of
               December 15, 2004 (as amended, supplemented or otherwise modified
               from time to time, the "Credit Agreement"), among WATERS
               CORPORATION, a Delaware corporation (the "Company"); WATERS
               TECHNOLOGIES IRELAND LTD. and WATERS CHROMATOGRAPHY IRELAND LTD.,
               both companies organized under the laws of Ireland (the "European
               Borrowers"; the Company and the European Borrowers being
               collectively called the "Borrowers"); the Lenders (as defined
               therein); JPMORGAN CHASE BANK, N.A., as administrative agent (the
               "Administrative Agent"); and J.P. MORGAN EUROPE LIMITED, as
               London Agent.

          A. Capitalized terms used but not defined herein shall have the
meanings assigned to such terms in the Credit Agreement.

          B. The Company has requested that the Lenders amend the Credit
Agreement as set forth herein.

          C. The Required Lenders are willing to effect such amendment on the
terms and subject to the conditions of this Amendment.

          D. Accordingly, in consideration of the mutual agreements herein
contained and other good and valuable consideration, the sufficiency and receipt
of which are hereby acknowledged, the parties hereto agree as follows:

          SECTION 1. Amendment of the Credit Agreement. Effective as of the
Amendment Effective Date:

          (a) Section 6.07 of the Credit Agreement is amended by deleting such
Section and replacing it with the following:

               "SECTION 6.07. Leverage Ratio. Permit the Leverage Ratio at any
     time to exceed 3.25:1.00."

          (b) The definition of "Applicable Rate" in Section 1.01 of the Credit
Agreement is amended by (i) deleting the table in paragraph (b) of such
definition and replacing it with the following table:

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                                                                               2


<TABLE>
<CAPTION>
                                       FACILITY
                                      FEE (BASIS    LIBOR SPREAD     ABR SPREAD
                    LEVERAGE          POINTS PER   (BASIS POINTS   (BASIS POINTS
 CATEGORY             RATIO             ANNUM)       PER ANNUM)      PER ANNUM)
----------   ----------------------   ----------   -------------   -------------
<S>          <C>                      <C>          <C>             <C>
Category 1   < 1.00                       8.0           29.5            0.0
Category 2   > or = 1.00 and < 1.50      10.0           40.0            0.0
Category 3   > or = 1.50 and < 2.00      12.5           50.0            0.0
Category 4   > or = 2.00 and < 2.50      15.0           60.0            0.0
Category 5   > or = 2.50 and < 3.00      17.5           80.0            0.0
Category 6   > or = 3.00                 20.0           92.5            0.0
</TABLE>

and (ii) replacing the words "Category 5" with the words "Category 6" in the
last sentence of paragraph (b) of such definition.

          (c) The definition of "Consolidated EBITDA" in Section 1.01 of the
Credit Agreement is amended by deleting such definition and replacing it with
the following:

          "Consolidated EBITDA" means, for any period, the consolidated net
     income (loss) of the Company and the Subsidiaries for such period plus, to
     the extent deducted in computing such consolidated net income for such
     period, the sum (without duplication) of (a) Consolidated Interest Expense,
     (b) consolidated income tax expense, (c) depreciation and amortization
     expense, (d) stock-based employee compensation expense related to any grant
     of stock options or restricted stock to the extent deducted from such
     consolidated net income for such period pursuant to Statement of Financial
     Accounting Standards 123 (revised 2004) and (e) extraordinary or
     non-recurring non-cash expenses or losses, minus, to the extent added in
     computing such consolidated net income for such period, extraordinary
     gains, all determined on a consolidated basis.

          SECTION 2. Representations and Warranties. To induce the other parties
hereto to enter into this Amendment, the Company represents and warrants to each
of the Lenders and the Administrative Agent that, as of the Amendment Effective
Date (as defined in Section 3):

          (a) The representations and warranties set forth in Article III of the
Credit Agreement are, after giving effect to this Amendment, true and correct in
all material respects on and as of the Amendment Effective Date with the same
effect as though made on and as of the Amendment Effective Date, except to the
extent such representations and warranties expressly relate to an earlier date
(in which case they were true and correct in all material respects as of such
earlier date).

          (b) No Default or Event of Default has occurred and is continuing.

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                                                                               3


          SECTION 3. Effectiveness. This Amendment shall become effective as of
the first date (the "Amendment Effective Date") on which the following
conditions have been satisfied:

          (a) The Administrative Agent (or its counsel) shall have received duly
executed counterparts hereof that, when taken together, bear the signatures of
(i) the Administrative Agent, (ii) the Company and (iii) the Required Lenders.

          (b) The Administrative Agent shall have received a certificate of the
chief financial officer of the Company to the effect that the representations
and warranties set forth in Section 2 hereof are true and correct on and as of
the Amendment Effective Date.

The Administrative Agent shall notify the Company and the Lenders of the
Amendment Effective Date, and such notice shall be conclusive and binding.

          SECTION 4. Effect of Amendment. (a) Except as expressly set forth
herein, this Amendment shall not by implication or otherwise limit, impair,
constitute a waiver of or otherwise affect the rights and remedies of the
Lenders or the Administrative Agent under the Credit Agreement or any other Loan
Document, and shall not alter, modify, amend or in any way affect any of the
terms, conditions, obligations, covenants or agreements contained in the Credit
Agreement or any other provision of the Credit Agreement or of any other Loan
Document, all of which are ratified and affirmed in all respects and shall
continue in full force and effect. Nothing herein shall be deemed to entitle the
Company or any other Borrower to a consent to, or a waiver, amendment,
modification or other change of, any of the terms, conditions, obligations,
covenants or agreements contained in the Credit Agreement or any other Loan
Document in similar or different circumstances.

          (b) On and after the Amendment Effective Date, each reference in the
Credit Agreement to "this Agreement", "hereunder", "hereof", "herein", or words
of like import, and each reference to the Credit Agreement in any Loan Document,
shall be deemed a reference to the Credit Agreement as amended hereby. This
Amendment shall constitute a "Loan Document" for all purposes of the Credit
Agreement and the other Loan Documents.

          SECTION 5. Costs and Expenses. The Company agrees to reimburse the
Administrative Agent for its reasonable out of pocket expenses in connection
with this Amendment, including the reasonable fees, charges and disbursements of
counsel for the Administrative Agent.

          SECTION 6. Counterparts. This Amendment may be executed in any number
of counterparts and by different parties hereto in separate counterparts, each
of which when so executed and delivered shall be deemed an original, but all
such counterparts together shall constitute but one and the same instrument.
Delivery of any executed counterpart of a signature page of this Amendment by
facsimile transmission shall be as effective as delivery of a manually executed
counterpart hereof.

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                                                                               4


          SECTION 7. Applicable Law. THIS AMENDMENT SHALL BE CONSTRUED IN
ACCORDANCE WITH AND GOVERNED BY THE LAW OF THE STATE OF NEW YORK.

          SECTION 8. Headings. The headings of this Amendment are for purposes
of reference only and shall not limit or otherwise affect the meaning hereof.

<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed by their duly authorized officers, all as of the date and year
first above written.

                                        WATERS CORPORATION,


                                        by /s/ John Ornell
                                           -------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------


                                        JPMORGAN CHASE BANK, N.A.,
                                        Individually and as Administrative
                                        Agent,


                                        by /s/ D. Scott Farquhar
                                           -------------------------------------
                                        Name: D. Scott Farquhar
                                        Title: Vice President

<PAGE>

                                                               SIGNATURE PAGE TO
                                                          FIRST AMENDMENT TO THE
                                             WATERS CORPORATION CREDIT AGREEMENT
                                                   DATED AS OF DECEMBER 15, 2004

To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

ABN AMRO BANK N.V.

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


by /s/ Eric Oppenheimer
   ----------------------------------
Name: Eric Oppenheimer
Title: Director


by /s/ Kevin Legallo
   ----------------------------------
Name: Kevin Legallo
Title: Assistant Vice President

<PAGE>

                                                                               2


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

Bank of America, N.A.

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


by  /s/ Peter D. Griffith
    ---------------------------------
Name: Peter D. Griffith
Title: Senior Vice President

<PAGE>

                                                                               3


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

Citizens Bank of Massachusetts

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


by  /s/ Paul R. Elwood
    ---------------------------------
Name: Paul R. Elwood
Title: Assistant Vice President

<PAGE>

                                                                               4


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

Barclays Bank PLC

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


by  /s/ David Barton
    ---------------------------------
Name: David Barton
Title: Associate Director

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                                                                               5


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

The Bank of New York

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


by /s/ Thomas J. McCormack
   ----------------------------------
Name: Thomas J. McCormack
Title: VP

<PAGE>

                                                                               6


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

Deutsche Bank AG New York Branch

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


by /s/ Frederick W. Laird
   ----------------------------------
Name: Frederick W. Laird
Title: Managing Director


by /s/ Ming K. Chu
   ----------------------------------
Name: Ming K. Chu
Title: Vice President

<PAGE>

                                                                               7


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

The Governor and Company of
the Bank of Ireland

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


by /s/ Fergus McDonald
   ----------------------------------
Name: Fergus McDonald
Title: Director

<PAGE>

                                                                               8


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

Bank of Tokyo-Mitsubishi
Trust Company

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


by /s/ A. Bernstein
   ----------------------------------
Name: A. Bernstein
Title: Assistant Vice President

<PAGE>

                                                                               9


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

HSBC Bank USA, N.A.

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


by /s/ Kenneth V. McGraime
   ----------------------------------
Name: Kenneth V. McGraime
Title: Senior Vice President,
       Commercial Executive

<PAGE>

                                                                              10


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

KEYBANK NATIONAL ASSOCIATION

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


by /s/ J. T. Taylor
   ----------------------------------
Name: J. T. Taylor
Title: Senior Vice President

<PAGE>

                                                                              11


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

Allied Irish Banks, plc.

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


by /s/ Michael Doyle
   ----------------------------------
Name: Michael Doyle
Title: Senior Vice President

<PAGE>

                                                                              12


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

Mizuho Corporate Bank (USA)

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


by /s/ Raymond Ventura
   ----------------------------------
Name: Raymond Ventura
Title: Senior Vice President

<PAGE>

                                                                              13


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

SunTrust Bank

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


by /s/ W. Brooks Hubbard
   ----------------------------------
Name: W. Brooks Hubbard
Title: Director

<PAGE>

                                                                              14


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

KBC Bank N.V.

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


by /s/ Robert M. Surdam, Jr.
   ----------------------------------
Name: Robert M. Surdam, Jr.
Title: Vice President


by /s/ Robert Snauffer
   ----------------------------------
Name: Robert Snauffer
Title: First Vice President

<PAGE>

                                                                              15


To approve First Amendment to the
Waters Corporation Credit Agreement
dated as of December 15, 2004:

Name of Institution:

The Norinchukin Bank, New York Branch

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


by /s/ Masanori Shoji
   ----------------------------------
Name: Masanori Shoji
Title: Joint General Manager