<PAGE>

                                                                   EXHIBIT 10.21


                                SECOND AMENDMENT

        SECOND AMENDMENT (this "Amendment"), dated as of February 7, 2000, among
Waters Corporation ("Holdings"), Waters Technologies Corporation (the
"Borrower"), the lenders listed on the signature pages hereof (the "Banks"), and
Bankers Trust Company, as Agent (in such capacity, the "Agent"). All capitalized
terms used herein and not otherwise defined herein shall have the respective
meanings provided for such terms in the Credit Agreement referred to below.

                              W I T N E S S E T H:


        WHEREAS, Holdings, the Borrower, the Banks of the Credit Agreement and
the Agent are parties to a Credit Agreement, dated as of November 22, 1995 and
amended and restated as of June 16, 1997 ( as further amended, modified and
supplemented to date, the "Credit Agreement"); and

        WHEREAS, the parties hereto wish to amend the Credit Agreement as herein
provided;


        NOW, THEREFORE, it is agreed:

        1. Section 6.05 of the Credit Agreement is hereby amended by (i)
deleting sub-clause (b) thereof in its entirety and (ii) inserting the following
new sub-clause (b) in lieu thereof:

        "(b) Neither the making of any Loan hereunder, nor the use of
        the proceeds thereof, will violate the provisions of
        Regulation T, U or X of the Board of Governors of the Federal
        Reserve System and no part of the proceeds of any Loan will be
        used to purchase or carry any Margin Stock or to extend credit
        for the purpose of purchasing or carrying any Margin Stock,
        provided that the Borrower may use the proceeds of Loans to
        make distributions or loans to Holdings to enable Holdings to
        purchase Margin Stock pursuant to Section 8.06(x) in
        compliance with Regulations T, U and X, so long as at the time
        of the making of such Loan, and after giving effect thereto,
        not more than 25% of the value of the assets of Holdings, the
        Borrower, or of the Borrower and its Subsidiaries on a
        consolidated basis, shall constitute Margin Stock."

        2. Section 6.13 of the Credit Agreement is hereby amended (i) by
inserting the text "As of the Restatement Effective Date," immediately before
the text "Holdings" appearing in the first sentence thereof and (ii) deleting
the text "Annex V" appearing twice therein and inserting the text "Annex IV" in
lieu thereof.


<PAGE>

        3. Section 6.17 of the Credit Agreement is hereby amended by (i)
deleting the text "Annex IV" appearing twice therein and (ii) inserting the text
"Annex V" in lieu thereof.

        4. Section 8.04 of the Credit Agreement is hereby amended by (i)
deleting sub-clause (e) thereof in its entirety and (ii) inserting the following
new text in lieu thereof:

        "(e)     Indebtedness  constituting  intercompany  loans  to  the
        extent  permitted  by  Section 8.05(g) and (y)".

        5. Section 8.05(v) of the Credit Agreement is hereby amended by
inserting the text "and Section 8.06(x)" immediately after the text "Section
8.06(vii)" appearing therein.

        6. Section 8.05 of the Credit Agreement is hereby further amended by (i)
deleting the text "and" appearing at the end of sub-clause (x) thereof, (ii)
deleting the text "(y)" appearing twice in sub-clause (y) thereof and replacing
it with the text "(z)", (iii) deleting the text "(x)" appearing in sub-clause
(y) thereof and replacing it with the text "(y)", (iv) inserting the text
"pursuant to this Section 8.05(z)" immediately before the period appearing at
the end of the clause that has been redesignated as sub-clause (z) pursuant to
sub-clause (ii) of paragraph 6 of this Amendment and (v) inserting the following
new sub-clause (y) immediately after sub-clause (x) appearing therein:

        "(y) So long as no Default or Event of Default then exists or would
        result therefrom, the Borrower may make loans and advances to Holdings,
        provided that (i) Holdings promptly thereafter uses the cash proceeds
        from such loans and advances for the purposes described in Section
        8.06(x) (such loans and advances, the "Repurchase Loans"), (ii) the sum
        of the aggregate amount of Repurchase Loans and Repurchase Dividends
        shall not exceed $200,000,000, (iii) such Repurchase Loans shall be
        evidenced by an Intercompany Note and (iv) each such Intercompany Note
        shall be pledged to the Collateral Agent pursuant to the Pledge
        Agreement; and"

        7. Section 8.06 of the Credit Agreement is hereby amended by (i)
deleting the text "and" appearing at the end of sub-clause (viii) thereof, (ii)
deleting the period appearing at the end of sub-clause (ix) thereof and
inserting a semi-colon in lieu thereof, and (iii) inserting the following new
sub-clauses (x) and (xi) immediately after sub-clause (ix):

        "(x) Holdings may redeem or purchase shares of Holdings Common Stock,
        provided that (i) at the time of any cash payment permitted to be made
        pursuant to this Section 8.06(x), no Default or Event of Default shall
        then exist or result therefrom and (ii) the aggregate amount of cash
        payments made pursuant to this Section 8.06(x) shall not exceed
        $200,000,000; and"

        "(xi) So long as no Default or Event of Default then exists or would
        result therefrom, the Borrower may pay cash Dividends to Holdings,
        provided that (i) Holdings promptly thereafter uses such cash proceeds
        for the purposes described

                                      -2-
<PAGE>
        in clause (x) of this Section 8.06 (such cash Dividends, the "Repurchase
        Dividends") and (ii) the sum of the aggregate amount of Repurchase
        Dividends and Repurchase Loans shall not exceed $200,000,000."

        8. Section 8.07 of the Credit Agreement is hereby amended by (i)
deleting the text "and" appearing at the end of clause (i) appearing therein and
inserting a comma in lieu thereof, (ii) inserting the text "Holdings"
immediately after the text "between or among" appearing in clause (ii) appearing
therein and (iii) inserting the following text immediately before the period
appearing at the end of said section:

        "and (iii) Holdings may redeem or purchase shares of Holdings Common
        Stock pursuant to Section 8.06(x) so long as Holdings determines in
        good faith that any such transactions with Affiliates are on
        commercially reasonable terms."

        9. Section 10 of the Credit Agreement is hereby amended by inserting the
following definitions in the appropriate alphabetical location:

        "'Repurchase Loans' shall have the meaning provided in Section 8.05(y).

        'Repurchase Dividends' shall have the meaning provided in Section
        8.06(xi)."

        10. Section 10 of the Credit Agreement is hereby further amended by
inserting the text "or intercompany loans as set forth in Section 8.05(y)"
immediately after the text "8.05(g)" appearing in the definition of
"Intercompany Note" appearing therein.

        11. Notwithstanding anything to the contrary contained in the Credit
Agreement, in the Pledge Agreement or in any other Credit Documents, the parties
hereto agree that Holdings will not be required to pledge to the Collateral
Agent (and neither the Collateral Agent nor any of the Banks shall have any Lien
on or in) any Holdings Common Stock constituting Margin Stock which is
repurchased by Holdings as described in Section 8.06(x) of the Credit Agreement
(after giving effect to this Amendment).

        12. In order to induce the Banks to enter into this Amendment, each of
Holdings and the Borrower hereby represents and warrants that (i) the
representations, warranties and agreements contained in Section 6 of the Credit
Agreement are true and correct in all material respects on and as of the
Amendment Effective Date (as defined in Section 17 of this Amendment after
giving effect thereto) (it being understood and agreed that any representation
or warranty which by its terms is made as of a specified date shall be required
to be true and correct in all material respects only as of such specified date)
and (ii) except as provided in paragraph 13 below, there exists no Default or
Event of Default on the Amendment Effective Date, both before and after giving
effect to this Amendment.

        13. The Banks hereby (i) waive the Event of Default caused by the
failure of the Borrower to comply with sub-clause (i) of the proviso appearing
in Section 8.12 of the Credit Agreement with respect to the new Subsidiaries
listed on Schedule 1 to this Amendment and (ii) waive for a period of 21 days
after the Amendment Effective Date, the Event of Default caused by the failure
of the Borrower to comply with the provisions of Section 8.12 of the Credit

                                      -3-
<PAGE>

Agreement (other than sub-clause (i) thereto) with respect to the new
Subsidiaries listed on Schedule 1 to this Amendment.

        14. This Amendment is limited as specified and shall not constitute a
modification, acceptance or waiver of any other provision of the Credit
Agreement or any other Credit Document.

        15. This Amendment may be executed in any number of counterparts and by
the different parties hereto on separate counterparts, each of which
counterparts when executed and delivered shall be an original, but all of which
shall together constitute one and the same instrument. A complete set of
counterparts shall be lodged with the Borrower and the Agent.

        16. THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES
HEREUNDER SHALL BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE LAW OF THE
STATE OF NEW YORK.

        17. This Amendment shall become effective on the date (the "Amendment
Effective Date") when each of Holdings, the Borrower and the Required Banks
shall have signed a counterpart hereof (whether the same or different
counterparts) and shall have delivered (including by way of facsimile
transmission) the same to the Agent at its address for notice provided for in
the Credit Agreement;

                                      * * *

                                       -4-
<PAGE>

        IN WITNESS WHEREOF, each of the parties hereto has caused a counterpart
of this Amendment to be duly executed and delivered as of the date first above
written.

                                       WATERS CORPORATION



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO

                                       WATERS TECHNOLOGIES CORPORATION



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                         Title: Senior Vice President and CFO

                                       WATERS FRANCE HOLDING CORP.



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO

                                       WATERS INVESTMENTS LIMITED



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO

                                       NIHON WATERS LIMITED



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO

                                       WFE HOLDING, INC.



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO
<PAGE>

                                       WATERS ASIA LIMITED



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO

                                       WATERS OPERATING CORP.



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO

                                       T.A. INSTRUMENTS, INC.



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO

                                       WATERS OPERATING COMPANY, L.L.C.



                                       By /s/ Philip S. Taymor
                                          --------------------------------------
                                          Title: Senior Vice President and CFO

                                       BANKERS TRUST COMPANY,
                                          Individually, as Agent
                                          and as Collateral Agent



                                       By /s/ Mary Kay Cole
                                          --------------------------------------
                                          Title: Managing Director

<PAGE>

                                       FLEET BANK, N.A.



                                       CREDIT INDUSTRIEL ET COMMERCIAL



                                       ABN AMRO BANK N.V.



                                       THE BANK OF NEW YORK



                                       THE BANK OF NOVA SCOTIA



                                       BANK OF SCOTLAND



                                      THE BANK OF TOKYO-MITSUBISHI TRUST COMPANY



                                      CREDIT AGRICOLE INDOSUEZ



                                      THE SAKURA BANK, LIMITED,
                                         NEW YORK BRANCH



                                      WACHOVIA BANK, N.A.



                                      FUJI BANK, LIMITED


<PAGE>
                                       GENERAL ELECTRIC CAPITAL CORPORATION



                                       THE INDUSTRIAL BANK OF JAPAN



                                       MELLON BANK, N.A.



                                       THE MITSUI TRUST & BANKING
                                         CO., LTD.



                                       ALLIED IRISH BANKS, P.L.C.,
                                         NEW YORK BRANCH



                                       CITIZENS BANK OF CONNECTICUT



                                       CITIZENS BANK OF MASSACHUSETTS



                                       THE ROYAL BANK OF SCOTLAND,
                                         PLC.



                                       SOCIETE GENERALE



                                       USTRUST