<PAGE>
                                                                   Exhibit 10.20

                      CHANGE OF CONTROL/SEVERANCE AGREEMENT

      This CHANGE OF CONTROL/SEVERANCE AGREEMENT, dated as of February 24, 2004,
is made by and between Waters Corporation (together with all subsidiaries or
affiliates hereinafter referred to as the "Company") and Douglas A. Berthiaume
(the "Executive").

      WHEREAS, the Executive has been hired as a senior executive of the Company
and is expected to make major contributions to the Company; and

      WHEREAS, the Company desires continuity of management; and

      WHEREAS, the Executive is willing to render services to the Company
subject to the conditions set forth in this Agreement.

      NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the Company and the Executive agree
as follows:

      1.    TERMINATION PRIOR TO A CHANGE OF CONTROL. If, within nine (9) months
prior to a Change of Control (as such term is defined in Section 3(c) below) and
subsequent to the commencement of substantive discussions that ultimately result
in the Change of Control, but prior to such Change of Control, the Company
terminates the Executive's employment with the Company for a reason other than
Cause (as such term is defined in Section 3(d) below), death or Disability (as
such term is defined in Section 3(e) below), the Company shall:

      (a)   Pay to the Executive a lump sum amount (reduced by any required
withholding), within ten (10) business days following the Change of Control,
equal to the sum of (i) twelve (12) times his monthly base salary (at the
highest monthly base salary rate in effect for the Executive in the twelve-month
period prior to the termination of his employment) and (ii) an amount equal to
the amount payable pursuant to the immediately preceding clause (i) times his
target bonus percentage under the Company's Management Incentive Plan or any
successor plan for the year in which the termination of the Executive's
employment occurs; and

      (b)   Provide the Executive and his dependents with the same life,
accident, health and dental insurance benefits that the Executive was receiving
immediately prior to the termination of his employment until the earlier of: (i)
the date which is twelve (12) months following the date of the Change of
Control; or (ii) the date the Executive commences subsequent employment;
provided, that if the Executive's continued participation is not possible under
the terms of any one or more of those insurance plans, the Company shall pay to
the Executive the amount the Company would have paid in premiums under the
relevant plan or plans had the Executive continued to be employed by the Company
and continued to participate in the relevant

                                       1
<PAGE>
plan or plans. The Executive and his dependents shall be entitled to health
insurance continuation coverage pursuant to the Consolidated Omnibus Budget
Reconciliation Act of 1985 ("COBRA"), from the date of discontinuance specified
in the preceding sentence, to the extent such coverage is required to be
provided in accordance with applicable law; and

      (c)   On the Change of Control, and notwithstanding any contrary
provisions of the Amended and Restated 1994 Stock Option Plan, the Second
Amended and Restated 1996 Long-Term Performance Incentive Plan or the 2003
Equity Incentive Plan (or any plans that may become the successors to such
plans) and any equity incentive agreements entered into between the Company and
the Executive pursuant to such plans or otherwise, cause any unexercisable
installments of any equity of the Company or any subsidiary or affiliate of the
Company held by the Executive pursuant to any such equity incentive agreement on
the Executive's last date of employment with the Company that have not expired
to become exercisable, or in the case of any restrictions on the vesting of any
equity of the Company or any subsidiary or affiliate of the Company held by the
Executive pursuant to any such equity incentive agreement, to cause such
restrictions to lapse, as the case may be, on the Change of Control; and

      (d)   On the Change of Control, cause any unvested portion of any
qualified or non-qualified capital accumulation benefits granted to the
Executive under the Waters Investment Plan, Waters Retirement Plan, Waters
401(k) Restoration Plan, and the Waters Retirement Restoration Plan (or any
plans that may become the successors to such plans) to become immediately vested
(subject to applicable law);

provided, however, that any amounts and benefits set forth in this Section 1
shall be reduced by any and all other severance or other amounts or benefits
paid or payable to the Executive as a result of the termination of his
employment.

      2.    TERMINATION FOLLOWING A CHANGE OF CONTROL.

      If, at any time during a period commencing with a Change of Control and
ending eighteen (18) months after such Change of Control, the Company terminates
the Executive's employment for a reason other than Cause, death, or Disability
or the Executive terminates his employment with the Company for "Good Reason"
(provided, however, that any such termination by the Executive must occur
promptly (and, in any event, within 90 days) after the occurrence of the event
or events constituting "Good Reason"), the Company shall:

      (a)   Pay to the Executive a lump sum amount (reduced by any required
withholding), within ten (10) business days following the Executive's last date
of employment, equal to the sum of twelve (12) times his monthly base salary (at
the highest monthly base salary rate in effect for such Executive in the twelve
(12) month period prior to the termination of his employment) and (ii) an amount
equal to the amount payable pursuant to the immediately preceding clause (i)
times his target bonus percentage under the Company's Management Incentive Plan
or any successor plan for the year in which the termination of the Executive's
employment occurs; and

      (b)   Provide the Executive and his dependents with the same life,
accident,

                                       2
<PAGE>
health and dental insurance benefits that the Executive was receiving
immediately prior to the termination of his employment until the earlier of: (i)
the date which is twelve (12) months following the date of the Change of
Control; or (ii) the date the Executive commences subsequent employment;
provided, that if the Executive's continued participation is not possible under
the terms of any one or more of those insurance plans, the Company shall pay to
the Executive the amount the Company would have paid in premiums under the
relevant plan or plans had the Executive continued to be employed by the Company
and continued to participate in the relevant plan or plans. The Executive and
his dependents shall be entitled to health insurance continuation coverage
pursuant to the Consolidated Omnibus Budget Reconciliation Act of 1985
("COBRA"), from the date of discontinuance specified in the preceding sentence,
to the extent such coverage is required to be provided in accordance with
applicable law; and

      (c)   Notwithstanding any contrary provisions of the Amended and Restated
1994 Stock Option Plan, the Second Amended and Restated 1996 Long-Term
Performance Incentive Plan or the 2003 Equity Incentive (or any plans that may
become the successors to such plans) and any equity incentive agreements entered
into between the Company and the Executive pursuant to such plans or otherwise,
cause any unexercisable installments of any equity of the Company or any
subsidiary or affiliate of the Company held by the Executive pursuant to any
such equity incentive agreement on the Executive's last date of employment with
the Company that have not expired to become exercisable, or, in the case of any
restrictions on the vesting of any equity of the Company or any subsidiary or
affiliate of the Company held by the Executive pursuant to any such equity
incentive agreement, to cause such restrictions to lapse, as the case may be, on
such last date of employment; and

      (d)   Cause any unvested portion of any qualified and non-qualified
capital accumulation benefits granted to the Executive under the Waters
Investment Plan, Waters Retirement Plan, Waters 401(k) Restoration Plan, and the
Waters Retirement Restoration Plan (or any plans that may become the successors
to such plans) to become immediately vested (subject to applicable law);

provided, however, that any amounts and benefits set forth in this Section 2
shall be reduced by any and all other severance or other amounts or benefits
paid or payable to the Executive as a result of the termination of his
employment.

      For purposes of this Section 2 above, "Good Reason" shall mean the
occurrence (without the Executive's express written consent) of one or more of
the following events following a Change of Control, as the case may be:

      (a)   The assignment to the Executive of any duties inconsistent in any
adverse, material respect with his position, authority, duties or
responsibilities immediately prior to the Change of Control or any other action
by the Company which results in a material diminution in such position,
authority, duties or responsibilities;

      (b)   A material reduction in the aggregate of the Executive's base and
incentive compensation (except for salary reductions or incentive compensation
reductions similarly affecting all senior executives of the Company) or the

                                       3
<PAGE>
termination of the Executive's rights to any employee benefits that he was
entitled to immediately prior to the Change of Control, except to the extent any
such benefit is replaced with a comparable benefit, or a reduction in scope or
value thereof; or

      (c)   A relocation of the Executive's place of business which results in
the one-way commuting distance for the Executive increasing by more than 25
miles from the location thereof immediately prior to the Change of Control
(provided, however, that travel with past practices for business purposes shall
not be considered "commuting" for purposes of this clause (iii)), or

      (d)   A failure by the Company to obtain the agreement referenced in
Section 3(h); provided, that the occurrence of any of the events listed in
clauses (a) though (d) shall not mean "Good Reason" if such event follows an
event or action by the Executive that would constitute Cause (as defined herein)
for termination.

      3.    GENERAL.

      (a)   Notwithstanding any other provision of this Agreement to the
contrary, benefits shall be payable under this paragraph only if the Executive
enters into a final and binding agreement prepared by the Company whereby the
Executive releases the Company and its subsidiaries (and those affiliated with
the Company and its subsidiaries) from all claims that the Executive may
otherwise have against them, to the extent that the basis for such claims arose
on or before the date the release is signed by the Executive; except that such
release shall not adversely affect the Executive's rights to enforce the terms
of this Agreement, and shall not adversely affect the Executive's right to any
indemnification or right to reimbursement of expenses by the Company to which he
would otherwise be entitled to under, without limitation, any charter document
or Company insurance policy, by reason of services he rendered for the Company
or any of its subsidiaries as an officer and/or an employee thereof.

      (b)   In the event the Executive's employment with the Company is
terminated by the Company for "Cause", or the Executive terminates his
employment with the Company other than during the specific time periods set
forth in Section 2 or for any reason other than Good Reason, the Executive shall
not be entitled to the severance benefits or other considerations described
herein by virtue of this Agreement.

      (c)   For purposes of this Agreement, "Change of Control' shall mean (i)
the closing of a merger, consolidation, liquidation or reorganization of the
Company into or with another company or other legal person, after which merger,
consolidation, liquidation or reorganization the capital stock of the Company
outstanding prior to consummation of the transaction is not converted into or
exchanged for or does not represent more than 50% of the aggregate voting power
of the surviving or resulting entity; (ii) the direct or indirect acquisition by
any person (as the term "person" is used in Section 13(d)(3) or 14(d)(2) of the
Securities Exchange Act of 1934, as amended) of more than 50% of the voting
capital stock of the Company, in a single or series of related transactions;
(iii) the sale, exchange, or transfer of all or substantially all of the
Company's assets (other than a sale, exchange, or transfer to one or more

                                       4
<PAGE>
entities where the stockholders of the Company immediately before such sale,
exchange or transfer retain, directly or indirectly, at least a majority of the
beneficial interest in the voting stock of the entities to which the assets were
transferred).

      (d)   For purposes of this Agreement, "Cause" shall mean: (i) the
conviction of the Executive by a court of competent jurisdiction of, or the
pleading of guilty or nolo contendere to, any felony or any crime involving
moral turpitude; (ii) gross negligence, breach of fiduciary duty or breach of
any confidentiality, non-competition or developments agreement in favor of the
Company; (iii) the Executive shall have willfully and continually failed to
substantially perform the Executive's duties with the Company after a written
demand for substantial performance is delivered by the Board, which demand
specifically identifies the manner in which the Board believes that the
Executive has not substantially performed the Executive's duties pursuant to the
disciplinary procedures of the Company, and such failure of substantial
performance shall have continued for a period of thirty (30) days after such
written demand, (iv) the Executive has been chronically absent from work
(excluding vacations, illnesses or leaves of absences), (iv) the commission by
the Executive of an act of fraud, embezzlement or misappropriation against the
Company; or (vi) the Executive shall have refused, after explicit notice, to
obey any lawful resolution or direction by the Board which is consistent with
his duties as an officer of the Company.

      (e)   For purposes of this Agreement, "Disability" means an independent
medical doctor (selected by the Company's health or disability insurer) has
certified that the Executive has, for six (6) months consecutive or
nonconsecutive in any 12 month period been disabled in a manner that seriously
interferes with his ability to perform his responsibilities as an employee of
the Company. Any refusal by the Executive to submit to a medical examination for
the purpose of certifying disability shall be deemed to constitute conclusive
evidence of the Executive's disability.

      (f)   Notwithstanding anything to the contrary in this Agreement, if any
portion of any payments received by the Executive from the Company (whether
payable pursuant to the terms of this Agreement or any other plan, agreement or
arrangement with the Company, its successors or any person whose actions result
in a Change of Control of the Company) shall be subject to tax imposed by
Section 4999 of the Internal Revenue Code of 1986, as amended, or any successor
statutory provision, the Company shall pay to the Executive such additional
amounts as are necessary so that, after taking into account any tax imposed by
Section 4999 (or any successor statutory provision), and any federal and state
income taxes payable on any such tax, the Executive is in the same after-tax
position that he would have been if Section 4999 (or any successor statutory
provision) did not apply and no payments were made pursuant to this Section
3(f). All determinations to be made under this Section 3(f), including whether a
gross-up payment is required and the amount of such gross-up payment, shall be
made by the Company, after consultations with its tax and accounting advisors.

      (g)   The parties hereto expressly agree that the payments by the Company
to the Executive in accordance with the terms of this Agreement will be
liquidated damages, and that the Executive shall not be required to mitigate the
amount of any payment provided for in this Agreement by seeking other employment
or otherwise,

                                       5
<PAGE>
nor shall any profits, income, earnings or other benefits from any source
whatsoever create any mitigation, offset, reduction or any other obligation on
the part of the Executive.

      (h)   Except as otherwise provided herein, this Agreement shall be binding
upon and inure to the benefit of the Company and any successor (whether direct
or indirect, by purchase, merger, consolidation, reorganization or otherwise) of
the Company. The Company shall require any such successor to assume this
Agreement expressly and to be bound by the provisions of this Agreement as if
such successor were the Company and for purposes of this Agreement, any such
successor of the Company shall be deemed to be the "Company" for all purposes.

      (i)   Nothing in this Agreement shall create any obligation on the part of
the Company or any other person to continue the employment of the Executive, and
nothing herein shall affect the Executive's obligations under any
non-competition, confidentiality, option or similar agreement between the
Company and the Executive currently in effect or which may be entered into in
the future.

      (j)   All payments required to be made by the Company hereunder to the
Executive shall be subject to the withholding of such amounts, if any, relating
to tax and other payroll deductions as the Company may reasonably determine it
must withhold pursuant to any applicable law or regulation.

      (k)   Any controversy or claim arising out of or relating to this
Agreement, or the breach thereof, shall be settled exclusively by
single-arbitrator arbitration in Boston, Massachusetts in accordance with the
Commercial Arbitration Rules of the American Arbitration Association then in
effect, and judgment upon the award rendered by the arbitrator may be entered in
any court having jurisdiction thereof.

      (l)   This Agreement shall be governed by and construed in accordance with
the laws of the Commonwealth of Massachusetts. This Agreement constitutes the
entire Agreement between the Executive and the Company concerning the subject
matter hereof and supercedes any prior negotiations, understandings, or
agreements concerning the subject matter hereof, whether oral or written, and
may be amended or rescinded only upon the written consent of the Company and the
Executive. The invalidity or unenforceability of any provision of this Agreement
shall not affect the other provisions of this Agreement and this Agreement shall
be construed and reformed to the fullest extent possible. The Executive may not
assign any of his rights or obligations under this Agreement; the rights and
obligations of the Company under this Agreement shall inure to the benefit of,
and shall be binding upon, the successors and assigns of the Company. This
Agreement may be executed in any number of counterparts, all of which taken
together shall constitute one and the same instrument.

                                       6
<PAGE>
IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be executed
as of the date first written above.

WATERS CORPORATION

By: /s/ Thomas P. Salice
    --------------------
     Thomas P. Salice
     Chairman, Compensation and Management Development Committee

THE EXECUTIVE

By: /s/ Douglas A. Berthiaume
    -------------------------
     Douglas A. Berthiaume

                                       7