<PAGE>

                                                                   Exhibit 10.18




                     SEPARATION REVOLVING CREDIT AGREEMENT



                            Dated:  March 31, 2001


                                    between



                             MILLIPORE CORPORATION

                                   as Lender



                                      and



                             MYKROLIS CORPORATION

                                  as Borrower
<PAGE>

THIS SEPARATION REVOLVING CREDIT AGREEMENT dated March 31, 2001, between

MILLIPORE CORPORATION, a Massachusetts corporation with a principal place of
business at 80 Ashby Road, Bedford, MA 01730 (the "Lender") and

MYKROLIS CORPORATION, a Delaware corporation with a principal place of business
at Patriots Park, Bedford, MA 01730 (the "Borrower")

IT IS  AGREED:

1.   The Lender shall establish the following revolving credit loan (the
     "Credit") in favor of the Borrower on the following terms:

       Credit Amount and Currency:   U.S.$100,000,000.00

       Drawdown Date:                From time to time at the request of the
                                     Borrower until the below specified
                                     Repayment Date, but only in multiples of
                                     U.S.$ 100,000.

       Repayment Date:               The earlier of fifteen (15) business days
                                     following the IPO Closing Date (as defined
                                     in the Master Separation and Distribution
                                     Agreement dated as of February   , 2001),
                                     or December 31, 2002.

       Interest Rate:                Until the Repayment Date 0% per annum,
                                     thereafter 1.5% per annum above 12 month
                                     LIBOR, to be adjusted semi-annually on the
                                     first days of January and July of each year
                                     or if any such day is not a Business Day,
                                     the immediately following Business Day

       Interest Payment Date:        Semi-annually on the first days of January
                                     and July of each year or if any such day is
                                     not a Business Day, the immediately
                                     following Business Day.

     For these purposes:

     "Business Day" means a day in which banks are open for business in Dublin
     and London.

     "LIBOR" means the rate offered by ABN AMRO Bank N.V. to prime banks in the
     London Interbank Market at or about 11:00 a.m. (London time) for twelve
     months U.S. Dollar deposits on the second Business Day before the first day
     of the relevant interest period, (being each period from one draw down date
     or anniversary thereof to the next anniversary thereof (as adjusted for
     Business Days)) or, if no such rate is available, at such time as
     reasonably determined by the Lender after consultation with ABN AMRO Bank
     N.V.

                                     --1--
<PAGE>

2.   Subject to the terms and conditions of this Revolving Credit Loan
     Agreement, the Credit shall be available to the Borrower in whole or in
     part from time to time, provided that the Lender shall have no obligation
     to make any loan to the Borrower under the Credit after the Repayment Date.
     Except as provided in clause (iii) below, each borrowing under the Credit
     shall be made in whole multiples of U.S.$ 100,000.  Loans shall be effected
     at the principal office of the lender at such times before the Repayment
     Date as the Borrower may request by three (3) Business Days' notice to the
     Lender.  During the term of the Credit, the Borrower may borrow, prepay and
     reborrow in accordance with the provisions of this Revolving Credit Loan
     Agreement.

     (i)    Loans under the Credit shall be evidenced by a promissory note in
            the form of Annex A attached hereto (the "Credit Note"), payable to
            the order of the Lender, duly executed on behalf of the Borrower,
            dated the date of the first borrowing under the Credit. The
            principal on the Credit Note shall be payable on or before the
            Repayment Date and shall bear interest (computed on the basis of a
            365-day year) from the Repayment Date on the unpaid principal amount
            thereof at the Interest Rate, payable on the Interest Payment Dates,
            commencing with the first of such dates next succeeding the date of
            the Credit Note and continuing until the later of payment in full of
            the Credit Note or the Repayment Date.

     (ii)   On three (3) Business Days' written notice (via facsimile or
            registered mail) to Lender, the Borrower may at its option prepay
            the Credit Note in whole at any time, or in part, from time to time,
            without payment penalty or premium but with all accrued and unpaid
            interest. Each partial prepayment shall be made in integrals of
            U.S.$ 100,000.

     (iii)  Amounts due from the Borrower to the Lender or due from the Lender
            to the Borrower in respect of Net Retention Adjustments pursuant to
            Section 2.3 of the General Assignment and Assumption Agreement
            between the Lender and the Borrower, in whatever integral, shall be
            treated as loans or prepayments hereunder, respectively, and
            notation thereof shall be made on the Credit Note in accordance with
            clause (iv) below.

     (iv)   The Lender shall make notation on the Credit Note of the amount of
            each loan made under the Credit, each payment or prepayment, the
            date thereof, the balance remaining due on the Credit Note as the
            result of each loan or payment and the other information specified
            on Credit Note.

3.   If the Borrower shall not pay on the due date any amount payable hereunder,
     the Borrower shall pay to the Lender, without prejudice to any other rights
     of the Lender hereunder or at law, default interest at the per annum rate
     specified under Interest Rate in Section 1 above.

4.   The Borrower's obligation to make any and all payments hereunder shall be
     absolute and unconditional and shall not be affected by any event or
     circumstance, including without limitation (i) any set-off, counterclaim,
     defense or other right which the Borrower may have against the Lender or
     any other person for any reason whatsoever, or (ii) any other event or
     circumstance whatsoever, whether or not similar to any of the foregoing.
     The Borrower shall make all payments hereunder in the currency in which
     they are expressed to

                                     --2--
<PAGE>

     be due by 11:00 a.m. on the due date in the location of payment to the
     Lender's account and bank specified by the Lender in advance, in full, and
     free and clear of all taxes and other withholdings or deductions of
     whatsoever nature.

5.   If the Borrower shall (i) make any default hereunder which is not remedied
     within 21 days or (ii) admit in writing its inability to pay its debts
     generally as they fall due, or (iii) apply for, or consent to, the
     appointment of, or the taking of possession by, a receiver, trustee,
     liquidator or the like of itself, or of all or a substantial part of its
     assets or (iv) be adjudicated or declared bankrupt or insolvent by any
     competent authority or (v) submit or consent to any proceedings for any
     such action, adjudication or (vi) submit or consent to any proceedings for
     any such action, adjudication or declaration, then, the Lender may,
     notwithstanding any delay or previous waiver of the right to do so, by
     notice in writing to the Borrower require the Borrower forthwith to prepay
     the balance of the loan outstanding hereunder together with accrued but
     unpaid interest and any other amounts owing hereunder, whereupon the same
     shall become immediately due and payable on such date.

6.   The Borrower shall not create or permit to exist any charge, mortgage, lien
     or encumbrance whatsoever on any of its assets for money borrowed unless
     the Borrower shall first have granted to the Lender security for its
     obligations hereunder ranking at least pari passu therewith and (save for
     debts preferred by law) the obligations hereunder shall rank at least pari
     passu with all other liabilities of the Borrower.

7.   The Borrower shall on demand indemnify the Lender, and reimburse the Lender
     upon demand, for all costs, losses, expenses (including reasonable legal
     expenses) and liabilities which the Lender may sustain or incur as a
     consequence of the occurrence and continuance of any default hereunder or
     any prepayment of any loan under the Credit pursuant to Clause 4.

8.   The Borrower may not assign or transfer any of its rights or obligations
     under this Agreement without the prior written consent of the Lender.

9.   This Agreement shall in all respects be governed by, and construed in
     accordance with, the laws of the Commonwealth of Massachusetts as applied
     to transactions between Massachusetts residents taking place entirely
     within Massachusetts.

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


<TABLE>
MILLIPORE CORPORATION                          MYKROLIS CORPORATION
<S>                                            <C>

By:  /s/ Francis J. Lunger                     By:  /s/ Jean Marc Pandraud
   -------------------------------               -------------------------------
          Signature                                       Signature

Francis J. Lunger, Executive Vice President    Jean Marc Pandraud, President
--------------------------------------------   --------------------------------------------
              Name, Title                                   Name, Title
</TABLE>

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<PAGE>

                                                   ANNEX A
                                                   =======

                                PROMISSORY NOTE
U.S.$ ___________                                           ___________ 2001

          On ________________, FOR VALUE RECEIVED, MYKROLIS CORPORATION a
Delaware corporation with a principal place of business at Patriots Park,
Bedford, MA 01730, (the "Maker"), hereby promises to pay to MILLIPORE
CORPORATION, a Massachusetts corporation (the "Payee"), or order, the sum of
_______________ U.S. Dollars (U.S.$ ___________), or such lesser unpaid
principal amount then advanced.  After maturity this Note shall bear interest at
an annual rate equal to the greater of (a) the Base Rate plus 1.5% (on a 365-day
basis) on the amount of principal from time to time unpaid.  Interest shall be
payable on the last days of January and June, commencing on the first of such
dates following maturity and until paid in full.  As used herein the term "Base
Rate" shall mean the rate of interest from time to time charged by ABN AMRO Bank
N.V. to prime banks in the London Interbank Market at or about 11:00 am (London
Time) for twelve (12) month deposits in the currency in which the principal
amount hereof is denominated.  Changes in the rate of interest shall be
effective on the first business days of January and July of each year. Principal
and interest shall be payable in immediately available funds at the office of
the Payee, 80 Ashby Road, Bedford, MA 01730 and may be prepaid in accordance
with the provisions of clause 2 of the Separation Revolving Credit Loan
Agreement referred to below at any time or from time to time, in whole or in
part, without penalty or premium but with all accrued and unpaid interest.  The
Maker hereby waives presentment, demand, notice, protest and all other demands
and notices in connection with the delivery, acceptance, performance or
enforcement hereof.

          This Note is the Credit Note referred to in, and is subject to the
terms of, a Separation Revolving Credit Loan Agreement dated March 31, 2001
between the Maker and Payee.  Neither this reference to such agreement nor the
terms thereof shall impair the unconditional obligation of the Maker to pay the
principal and interest on this Note when due.

                                  Mykrolis Corporation

                                By:_____________________________
                                   Name, Title:



                               Payments                            Name of
             Amount      --------------------    Unpaid Principal  Person Making
     Date    of Loan     Principal   Interest     Balance of Note  the Notation
     ----    -------     ---------   --------     ---------------  ------------

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