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                                                                  Exhibit 10.11

                              EMPLOYMENT AGREEMENT

         THIS EMPLOYMENT AGREEMENT (the "Agreement") is made this 1st day of
December, 2003 between Idenix Pharmaceuticals, Inc., a Delaware corporation
(together with its successors and assigns, the "Company") and David Arkowitz
(the "Employee").

         WHEREAS, the Company desires to employ the Employee as the Company's
Chief Financial Officer;

         WHEREAS, the Employee is willing to accept such employment with the
Company; and

         WHEREAS, the Company and the Employee desire to enter into an
employment agreement upon the terms and conditions set forth herein.

         NOW, THEREFORE, in consideration of the mutual covenants and
representations contained herein, the Company and the Employee (individually, a
"Party" and, together, the "Parties") hereto agree as follows.

1.       EMPLOYMENT PERIOD

         COMMENCING ON DECEMBER 1, 2003 (the "Effective Date"), the Company
shall employ the Employee, and the Employee shall serve the Company, under the
terms of this Agreement for a term (the "Term") of three (3) years from the
Effective Date (the "Initial Term"), provided, however, that the Term shall
automatically renew for successive one-year terms, unless either Party gives
written notice to the other party not less than 120 calendar days prior to the
expiration of any such term that such Party is electing not to so extend the
Term. Notwithstanding the foregoing, the Employee's employment hereunder may be
terminated at any time, subject to Section 4 and Section 5 hereof, following the
Effective Date. The period of time during which the Employee is employed
hereunder shall be referred to herein as the "Employment Period."

2.       DUTIES AND STATUS; PLACE OF EMPLOYMENT

         During the Employment Period, the Company hereby engages the Employee
as the Company's Chief Financial Officer on the terms and conditions set forth
in this Agreement. During the Employment Period, the Employee shall report to
the Company's Chief Executive Officer, and shall exercise such authority and
perform such duties and functions and discharge such responsibilities as are
customarily associated with the Employee's position and shall not be assigned
duties that are materially inconsistent with his ability to discharge the
foregoing duties and responsibilities. During the Employment Period, the
Employee agrees to devote substantially all of the Employee's business time,
efforts and skills to the performance of the Employee's duties and
responsibilities under this Agreement, provided, however, that the Employee
shall be permitted to manage his personal investments and affairs, to serve with
the prior consent of the Company's Chief Executive Officer on such scientific
advisory boards or other similar organizations as agreed to promote the
interests of the Company and to engage in charitable activities and community
affairs provided such activities do not materially interfere with the proper
performance of the Employee's duties and responsibilities hereunder. During the

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Employment Period, the Employee's principal place of employment shall be in the
Boston, Massachusetts metropolitan area.

3.       COMPENSATION AND BENEFITS

         A.       Salary. During the Employment Period, the Company shall pay to
the Employee, as compensation for the performance of the Employee's duties and
obligations under this Agreement, an annual base salary of $290,000 ("Base
Salary"), payable in accordance with the normal payroll practices of the
Company. The Base Salary shall be reviewed annually for additional increases, if
any, which in the sole discretion of the Board of Directors of the Company (the
"Board") are merited or necessary to adjust for cost of living increases or to
retain a competitive Base Salary for the Employee. After any such increase, the
term "Base Salary" as utilized in this Agreement shall thereafter refer to the
increased amount. Base Salary shall not be reduced at any time without the
express prior written consent of the Employee.

         B.       Annual Bonus. During the Employment Period, the Employee shall
be eligible for an annual bonus equal to no less than 30% of the Employee's Base
Salary (the "Target Bonus"), based upon objective and measurable criteria which
the Board shall establish annually after consultation with Employee and
Company's senior management, which criteria shall be communicated to the
Employee in writing prior to the start of the applicable performance period. The
Target Bonus shall be payable in such amount if the relevant criteria are met.
If such performance goals are exceeded, the Employee shall receive a larger
amount on a sliding scale of up to 200% of the annual target bonus opportunity.
If such performance goals are not reached, the Employee shall receive a smaller
amount on a sliding scale of from 0% to 100% of the annual target bonus
opportunity. Any annual bonus shall be payable in accordance with the terms of
the bonus program established by the Board after consultation with Employee and
the Company's Senior Management, provided that any bonus earned shall be paid no
later than 90 calendar days after the end of the applicable performance period.
The Employee's target bonus opportunity as a percentage of Base Salary may be
reviewed periodically for any increase in the discretion of the Board. After any
such increase, the term "Target Bonus" as utilized in this Agreement shall
thereafter refer to the increased amount. The Target Bonus as a percentage of
Base Salary shall not be reduced, either during a given year, or from one year
to the next, without the express prior written consent of the Employee.

         C.       Sign-on Bonus. As a further inducement to accept employment
with the Company, the Company shall pay to the Employee the amount of $200,000
(the "SignOn Bonus") not later than 90 calendar days after the commencement of
the Employment Period. Under no circumstances shall Employee be liable to repay
any portion of the Sign-On Bonus, except that if Employee's employment is
terminated prior to the 2nd anniversary of the Employment Period because of
either a termination by Employee without Good Reason or a termination by
Employer for Cause, Employee shall in those instances only be liable for
repayment of a prorata portion of the Sign-on Bonus. The prorata amount
repayable will be equal to the aggregate Sign-on Bonus ($200,000) less the
aggregate monthly reductions (in the amount of $8,333 per month) effected prior
to the termination of the Employment Period. The monthly reductions will be
effected on the last day of each month during the 1st and 2nd year of the
Employment Period, beginning with the first month of the Employment Period, even
if the Sign-On Bonus is not paid until a later month. Any amounts the Employee
is obligated to repay

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under this Section 3.C. will be due and payable within 90 days of the
termination of the Employment Period.

         D.       Equity.

                  (i)      Equity Award - Commencement of Employment Period. In
connection with and upon the commencement of the Employment Period, the Employee
will be awarded, pursuant to the Company's 1998 Equity Incentive Plan, as
amended (as the same may be further amended, modified or supplemented, the "1998
Plan") an option (the "Employment Commencement Option") to purchase 175,000
shares of the Company's common stock, par value $.001 per share (the "Common
Stock"). Such award will vest ratably on a monthly basis over the 48 months
subsequent to the date of grant.

                  (ii)     Vesting. Any unvested or restricted Equity Awards
will continue to vest (or have restrictions lapse) in accordance with the terms
of this Agreement or the applicable plan and award agreement, including any
provision providing for acceleration of vesting (or lapsing of restrictions)
upon any "change in control" of the Company (as defined in the applicable plan
or award agreement) that may occur following the Effective Date. For purposes
hereof, "Equity Awards" shall include the Employment Commencement Option, and
shall mean any options to purchase Common Stock or other shares of the Company's
capital stock, any restricted shares of Common Stock or other shares of the
Company's capital stock or any other equity-based awards of the Company granted
to the Employee.

                  (iii)    Additional Equity Awards. The Employee shall be
eligible to participate in any long-term incentive award program of the Company,
including, but not limited to, the 1998 Plan and any successor thereto. In
addition, commencing with the year ending December 31, 2003, the Employee shall
have an annual performance equity opportunity to be awarded an option to
purchase not less than 30,000 shares of the Common Stock (with such award
vesting ratably over the 48 months following the date of grant) (the "Annual
Equity Bonus"). The partial year of service the Employee will render to the
Company in 2003 will not affect, diminish, reduce or result in any pro-ration of
such Annual Equity Bonus for the year ending December 31, 2003. The opportunity
to be awarded an Annual Equity Bonus shall be afforded to Employee each year,
with the actual number of shares to be awarded under such opportunity subject to
annual approval by the Company's Board of Directors and conditioned upon the
achievement of annual performance targets established by the Board in
consultation with Employee and Company's senior management. The performance
targets for the Annual Equity Bonus shall be objective and measurable and shall
be communicated to Employee in writing prior to the start of the applicable
bonus period.

         E.       Employee Benefits. During the Employment Period, the Employee
shall be entitled to participate in all of the employee and fringe benefit plans
of the Company in effect during the Employment Period on the same basis as
provided generally to other senior-level executives of the Company, subject to,
and on a basis consistent with, the participation requirements and other terms
and conditions of such plans. The Employee shall be entitled to four (4) weeks
vacation per calendar year, subject to such carry-over policy as is approved by
the Board.

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         F.       Business Expenses. During the Employment Period, the Company
shall promptly reimburse the Employee for all appropriately documented,
reasonable business expenses incurred by the Employee in the performance of the
Employee's duties under this Agreement, in accordance with the Company's
policies.

         G.       Relocation. In connection with the Employee's relocation to
the Boston, Massachusetts metropolitan area, Idenix will reimburse the Employee
for reasonable moving and relocation expenses (not to exceed $25,000), real
estate commissions (not to exceed 6%) and any conveyancing fees and/or transfer
taxes incurred on the sale of the Employee's current principal residence, actual
expenses of two househunting trips to Cambridge for himself and his family,
temporary housing for three (3) months during the Initial Term (to be utilized
in the Employee's discretion), and amounts required to gross up the aggregate of
all of the foregoing to offset any related additional income tax liabilities.
Company shall have no payment or reimbursement obligation under this Section 3.G
for any expenditures or obligations incurred after the third anniversary of the
Employment Period commencement.

4.       TERMINATION OF EMPLOYMENT

         A.       Termination by the Company. The Company shall have the right
to terminate the Employment Period and the Employee's employment hereunder for
any reason at any time, including for any reason that does not constitute Cause,
subject to the consequences of such termination as provided in this Agreement.
For purposes of this Agreement, and subject to the Employee's opportunity to
cure as provided below in Section 4.C., the Company shall have "Cause" to
terminate the Employee's employment hereunder if such termination shall be the
result of: (i) willful fraud or willful material dishonesty in connection with
the Employee's employment by the Company; (ii) intentional failure by the
Employee to substantially perform the Employee's duties hereunder or gross
neglect in the performance of such duties; (iii) gross misconduct by the
Employee that is materially detrimental to the Company's reputation, goodwill or
business operations; (iv) a material breach of any of the Employee's covenants
as provided in Section 7 hereof; (v) a breach of any representation or warranty
made by the Employee in Section 7 hereof; (vi) an indictment or conviction on a
felony charge; or (vii) the Employee becoming subject to a civil, regulatory or
criminal proceeding which naming of the Employee results in material harm to the
reputation, business or prospects of the Company; or (viii) the entering of any
civil, regulatory or enforcement determination (whether pursuant to a formal or
informal agreement or court order between the prosecuting entity and Employee)
relating to the Employee's service as a director, officer, employee of any
entity, including but not limited to the Company, provided, however, that such
finding to the extent it relates to matters involving the Employee's employment
with the Company was not sanctioned or approved by Company.

         B.       Termination by the Employee. The Employee shall have the right
to terminate the Employment Period for any reason at any time, including for any
reason that does not constitute Good Reason, subject to the consequences of such
termination and the Employee's covenants as provided in this Agreement and such
termination shall not be deemed to be a breach of this Agreement. For purposes
of this Agreement, and subject to the Company's opportunity to cure as provided
below, the Employee shall have "Good Reason" to terminate the Employee's
employment hereunder if such termination shall be the result of the following,
without the Employee's prior written consent: (i) any adverse change in the
Employee's title, any change

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resulting in the Employee reporting to any person other than the Company's Chief
Executive Officer or any material diminution in the Employee's authority or
responsibilities as set forth in Section 2 hereof (other than as a result of a
court order or regulatory action involving the Employee, including a formal or
informal consent order or agreement between the prosecuting entity and
Employee), (ii) a reduction, either from one year to the next, or within the
current year, in the Employee's Base Salary, the Target Bonus (as a percentage
of Base Salary) or target Annual Equity Bonus, (iii) a relocation of the
Employee's primary place of employment to a location more than 40 miles from
Employee's then-current primary place of employment, (iv) any other material
breach by the Company of this Agreement, or (v) any act committed by Company in
bad faith or without legitimate business justification that causes material
injury to Employee or to his name, reputation, or professional or business
interests.

         C.       Notice and Opportunity to Cure. Notwithstanding the foregoing,
it shall be a condition precedent to the Company's right to terminate the
Employee's employment for Cause pursuant to clauses (i) through (v) of Section
4.A. above and the Employee's right to terminate the Employee's employment for
Good Reason that (1) the party seeking the termination shall first have given
the other party written notice stating with reasonable specificity the reason
for the termination ("breach"), (2) if such breach is susceptible of cure or
remedy, a period of 30 calendar days from and after the giving of such notice
shall have elapsed without the breaching party having effectively cured or
remedied such breach during such 30-day period and (3) in the case of a
termination for Cause, if the Employee fails to cure such neglect or conduct
within such 30-day period, the Employee has an opportunity to be heard before
the Board or, if the Board shall so designate, before a committee comprised of
the Chief Executive Officer of the Company and two other senior executives of
the Company, in each such case subject to a majority vote to terminate the
Employee's employment for Cause. Notwithstanding the foregoing, (a) Company
shall not be required to provide notice and opportunity to cure if Employee
commits an act constituting Cause that is the same as or substantially similar
to a prior act constituting Cause and for which Company provided a termination
notice, and (b) Employee shall not be required to provide notice and opportunity
to cure if Company commits an act giving rise to Good Reason that is the same as
or substantially similar to a prior act constituting Good Reason and for which
Employee provided a termination notice.

         D.       Termination Upon Death or Disability. The Employment Period
and the Employee's employment hereunder shall be terminated by the death of the
Employee. The Employment Period and the Employee's employment hereunder may be
terminated by the Company or the Employee if the Employee shall be rendered
incapable, with or without reasonable accommodations by Company, of performing
the Employee's duties to the Company by reason of any medically determined
physical or mental impairment that has lasted or is reasonably expected to last
for a period of six (6) or more consecutive months from the first date of the
Employee's absence due to the disability (a "Disability").

         E.       Expiration of Term. The Employment Period shall terminate upon
the expiration of the Term. Any continuation of the Employee's employment beyond
the expiration of the original Term or any extended Term shall not be subject to
the terms and conditions of this Agreement, except that the "Inventions
Agreement" set forth as Exhibit A shall survive the Term and the expiration of
the Employment Period, and Sections 7 and 9 hereof shall survive in accordance
with their terms for the periods specified therein.

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5.       CONSEQUENCES OF TERMINATION

         A. Without Cause; For Good Reason. In the event of a termination of the
Employee's employment during the Employment Period by the Company other than for
Cause or by the Employee for Good Reason (each, a "Covered Termination"), the
Company shall provide the Employee, upon expiration of the revocation period in
the Employee Release (as defined in Section 8 below), with the following:

                  (i)      payment of a cash lump-sum amount equal to the sum of
         (x) the Employee's Base Salary and (y) the greater of (1) the current
         Target Bonus or (2) the bonus earned by the Employee for the year
         preceding the year in which termination of employment occurs, payable
         as soon as practicable but not later than forty-five (45) days after
         the expiration of the revocation period in the Employee Release;

                  (ii)     immediate vesting and exercisability (or lapsing of
         restrictions) of all outstanding equity awards, including any Equity
         Awards, upon a Covered Termination, with any equity awards that are
         stock options remaining exercisable for 24 months following the date of
         a Covered Termination (or until the expiration of the term of the
         option, if earlier); and

                  (iii)    continued coverage for the Employee and the
         Employee's eligible dependents under all group medical, dental and life
         insurance coverages that are provided to employees of the Company
         generally for a period of 12 months following a Covered Termination,
         with such coverage to be at the Company's cost (subject to standard
         employee contribution requirements). Any such coverage shall be
         discontinued in the event that the Employee obtains substitute coverage
         from subsequent employment or service during such 12-month period.

                  In addition, without the need for Employee to sign the
         Employee Release, Company shall pay Employee (w) the Sign-On Bonus, if
         not yet paid, (x) any earned but unpaid amounts as of the date of
         termination, including, but not limited to, Base Salary through the
         date of termination, reimbursement of business expenses and any
         incentive awards earned for performance periods that have ended,
         including annual bonus and Annual Equity Awards, (y) any compensation
         previously deferred by the Employee together with any vested Company
         matching contributions and (z) any accrued but unpaid vacation days
         under Company policy through the date of termination (all of which, (w)
         through (z), are referred to hereafter as "Accrued Obligations"),
         payable as soon as practicable following the termination of employment,
         but not later than forty-five (45) days thereafter.

         B.       Death or Disability. In the event the Employee's employment
with the Company is terminated on account of death or Disability during the
Employment Period, the Company shall provide the Employee (or his estate or
legal representative, as the case may be) with such death or disability benefits
as are provided under the death and disability plans that are available to
employees of the Company generally on the date of termination. In addition, the
Employee (or his estate or legal representative, as the case may be) shall be
entitled to:

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                  (i)      in the case of Disability, payment of 60% of Base
         Salary per year until the Employee reaches age 65 (with such payment
         obligation offset by any amounts the Employee receives from any
         insurance plan or policy of the Company);

                  (ii)     in the case of death, payment to Employee's estate of
         one-half (1/2) of the sum set forth in Section 5(A)(i) above as soon as
         practicable but not more than forty-five (45) days following Employee's
         death;

                  (iii)    in the case of Disability, if Employee dies within
         six (6) months of the termination of his employment as a result of the
         condition that caused him to be disabled, payment to Employee's estate
         of one-half (1/2) of the sum set forth in Section 5(A)(i) above, as
         soon as practicable but not more than forty-five (45) days following
         Employee's death;

                  (iv)     in the case of death or Disability, immediate vesting
         and exercisability (or lapsing of restrictions) of all outstanding
         equity awards, including any Equity Awards, upon such termination of
         employment, with any equity awards that are stock options remaining
         exercisable for 24 months following the date of termination (or until
         the expiration of the term of the option, if earlier);

                  (v)      continued coverage for the Employee (in the case of
         Disability) and the Employee's eligible dependents under all group
         medical, dental and life insurance coverages that are provided to
         employees of the Company generally for a period of 12 months following
         such termination, with such coverage to be at the Company's cost
         (subject to standard employee contribution requirements). Any such
         coverage shall be discontinued in the event that the Employee obtains
         substitute coverage from subsequent employment or service during such
         12-month period; and

                  (vi)     in the case of death or Disability, payment of all
         Accrued Obligations as soon as practicable but not more than forty-five
         (45) days following the termination of employment.

         C.       Other Termination.

                  (i)      In the event that the Employee's employment with the
Company is terminated during the Employment Period (i) by the Company for Cause,
or (ii) by the Employee other than for Good Reason, the Employee shall not be
entitled to any further payments, compensation or other benefits under this
Agreement, except the Employee shall be entitled to (i) payment of Accrued
Obligations as soon as practicable but not later than forty-five (45) days
following such termination, and (ii) treatment of any outstanding equity awards
in accordance with the applicable plan or award agreement.

                  (ii)     In the event that the Company elects not to extend
the Term beyond the Initial Term, to the extent that the Employment Commencement
Option or shares of Common Stock acquired by the Employee pursuant to the
exercise thereof remains unvested or subject to restriction intended to lapse
with the passage of time, (the "Vesting Restrictions"), such option shall
immediately vest and become exercisable in full for a period of 15 months after
the

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expiration of the Initial Term and Vesting Restrictions will immediately lapse
upon expiration of the Initial Term.

         D.       Other Benefits. The benefits payable to the Employee under
this Agreement are not in lieu of any benefits payable or entitlements due under
any applicable plan, program, policy or arrangement of, or other agreement with,
the Company or any affiliate, and upon termination the Employee shall receive
such benefits, payments or entitlements, if any, as he may be entitled to
receive pursuant to the terms of such plans, programs, policies, arrangements or
other agreements. Except for the obligations of the Company provided by the
foregoing and this Section 5, the Company shall have no further severance
obligations to the Employee or the Employee's beneficiaries upon the Employee's
termination of employment.

         E.       No Mitigation or Offset. The Employee shall have no obligation
to mitigate the damages provided by this Section 5 by seeking substitute
employment or otherwise and, except as provided in Section 5.A(iii) and Section
5.B(iii) hereof, there shall be no offset of the payments or benefits provided
in this Section 5. In addition, except as provided in Section 7.E hereof, the
Company's obligation to make any payment pursuant to, or otherwise perform its
obligations under, this Agreement shall not be affected by any claim or other
right the Company or any affiliate may have against the Employee for any reason.

6.       CHANGE IN CONTROL PROTECTIONS

         A.       Termination Within One Year After a Change in Control. If the
Employee's employment is terminated by the Company without Cause or the Employee
terminates his employment for Good Reason, in each case within one (1) year
following a Change in Control and during the Term, then, in addition to the
payments, benefits and entitlements provided pursuant to Section 5.A hereof, the
Employee shall also be entitled to an additional lump-sum amount equal to the
sum of (x) the Employee's Base Salary and (y) the greater of (1) Target Bonus or
(2) the bonus earned by the Employee for the year preceding the year in which
termination of employment occurs, payable as soon as practicable but in no event
more than forty-five (45) days following the termination of employment.

         B.       Excise Tax Provision. Anything herein to the contrary
notwithstanding, to the extent that any payment, entitlement or benefit provided
under this Agreement or any other agreement, plan, policy, program or
arrangement of the Company (the "Payments") would be subject to the imposition
of the excise tax imposed under Section 4999 of the Internal Revenue Code of
1986, as amended, or any similar Federal or state law (an "Excise Tax"), the
Payments shall be reduced (but not below zero) to the maximum amount as will
result in no portion of the Payments being subject to such Excise Tax (the "Safe
Harbor Cap"), but only if the net after-tax amount that would be received by the
Employee, taking into account all applicable Federal, state and local income
taxes and the imposition of the Excise Tax, is greater than the net after-tax
amount that would be received by the Employee if Payments are not reduced to the
Safe Harbor Cap. Unless the Employee has given prior written notice specifying a
different order to the Company to effectuate the reductions described in the
preceding sentence, the Company shall reduce or eliminate the Payments to the
Safe Harbor Cap, by first reducing or eliminating those payments or benefits
which are not payable in cash and then by reducing or eliminating cash payments.
Any notice given by the Employee pursuant to the preceding sentence shall take

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precedence over the provisions of any other plan, arrangement or agreement
governing the Employee's rights and entitlements to any benefit, entitlement or
compensation.

         C.       Definition of Change in Control. For purposes of Section 3
hereof and this Section 6, the term "Change in Control" shall mean:

                  (i)      any "person," as such term is used in Sections
         3(a)(9) and 13(d) of the Securities Exchange Act of 1934, becomes a
         "beneficial owner," as such term is used in Rule 13d-3 promulgated
         under that act, of fifty percent (50%) or more of the Voting Stock of
         the Company, or Novartis Pharma AG disposes of its entire interest in
         the Company's Voting Stock (except in connection with an initial public
         offering of the Voting Stock);

                  (ii)     the Company adopts any plan of liquidation providing
         for the distribution of all or substantially all of its assets;

                  (iii)    all or substantially all of the assets or business of
         the Company is disposed of pursuant to a merger, consolidation or other
         transaction (unless the shareholders of the Company immediately prior
         to such merger, consolidation or other transaction beneficially own,
         directly or indirectly, at least fifty percent (50%) of the Voting
         Stock or other ownership interests of the entity or entities, if any,
         that succeed to the business of the Company); or

                  (iv)     the Company combines with another company and is the
         surviving corporation but, immediately after the combination, the
         shareholders of the Company immediately prior to the combination hold,
         directly or indirectly, fifty percent (50%) or less of the Voting Stock
         of the combined company (there being excluded from the number of shares
         held by such shareholders, but not from the Voting Stock of the
         combined company, any shares received by affiliates of such other
         company in exchange for stock of such other company).

         For purposes of this definition of "Change in Control" the "Company"
shall include any entity that succeeds to all or substantially all of the
business of the Company and "Voting Stock" shall mean securities of any class or
classes having general voting power under ordinary circumstances, in the absence
of contingencies, to elect the directors of a corporation.

7.       REPRESENTATIONS AND COVENANTS

         A.       Representations of Employee. The Employee represents and
warrants that (i) he has the free and unfettered right to enter into this
Agreement and to perform his obligations under the term of this Agreement and
that the Employee knows of no agreement between him and any other person, firm
or organization, or any law or regulation that would be violated by the
performance of his obligations under this Agreement; (ii) he is not aware of any
regulatory, court or other proceeding relating to his service as a director,
officer or employee of any other entity; and (iii) he has not been charged with,
or indicted for, any crime.

         B.       Inventions. The Employee agrees to sign and honor an
"Inventions Agreement" in the form attached hereto as Exhibit A.

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         C.       Nonsolicitation of Employees. During the Restricted Period
(defined as the Employment Period and a period of one (1) year following the
termination of the Employment Period), the Employee shall not, without the
express prior written approval of the Company, (i) directly or indirectly,
knowingly solicit any employee of the Company or any of its affiliates (or any
employee who was employed by the Company or any of its affiliates at any time
within six (6) months prior to the date the Employee seeks to solicit such
person ("Former Employee")) to leave the employ of the Company or its
affiliates, as the case may be.

         D.       Nonsolicitation of Customers. During the Restricted Period,
the Employee agrees that other than in the ordinary course of business he shall
not, without the express prior written approval of the Company, knowingly
solicit, recruit or encourage any customer of the Company or any of its
affiliates who was a customer at, or was a customer within the six (6)-month
period preceding, the date of the Employee's termination of employment to reduce
or cease its business with the Company or any such affiliate. Provided Employee
is not himself involved in soliciting business, it shall not be a violation of
this paragraph for Employee, after the termination of his employment, to be
employed by or to provide services to a firm that competes with Company or that
solicits business from customers or former customers of Company.

         E.       Enforcement. The Employee acknowledges that if he breaches any
provision of this Section 7, the Company will suffer irreparable injury. It is
therefore agreed that the Company shall have the right, if permitted by a court
of the applicable jurisdiction, to enjoin any such breach, without posting any
bond. The Employee hereby waives the adequacy of a remedy at law as a defense to
such relief. The existence of this right to injunctive, or other equitable
relief, shall not limit any other rights or remedies which the Company may have
at law or in equity including, without limitation, the right to monetary,
compensatory and punitive damages. The Employee acknowledges and agrees that the
provisions of this Section 7 are reasonable and necessary for the successful
operation of the Company. In the event a court of competent jurisdiction
determines that the Employee has breached the Employee's obligations in any
material respect under this Section 7 (other than through the issuance of an
injunction issued without a determination on the merits), the Company, in
addition to pursuing all available remedies under this Agreement, at law or
otherwise, and without limiting its right to pursue the same, shall be entitled
to cease all payments due to the Employee under this Agreement as of the date of
such determination. In the event that a court of competent jurisdiction (or an
arbitrator, pursuant to Section 9 below) determines that the Company has failed
to pay any amount due Employee under this Agreement, then the rights of Employer
and the obligations of Employee set forth in this Section 7, including those set
forth in the Inventions Agreement, shall be null and void and of no further
effect as of the date of such determination.

8.       WAIVER

         All payments and benefits provided by Section 5.A hereof (except for
all Accrued Obligations) are conditioned upon the Employee executing and
honoring a release of claims in favor of the Company following the Employee's
termination of employment in a form attached hereto as Exhibit B (the "Employee
Release").

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9.       DISPUTE RESOLUTION

         Except as otherwise provided in Section 7.E hereof, any controversy,
dispute or claim arising out of or relating to this Agreement shall be resolved
by final and binding arbitration, to be held in Boston, Massachusetts, in
accordance with the Commercial Arbitration Rules (and not the National Rules for
the Resolution of Employment Disputes) of the American Arbitration Association
and this Section 9. Judgment upon the award rendered by the arbitrator(s) may be
entered in any court having jurisdiction thereof. Neither Party shall be liable
for punitive or exemplary damages, except to the extent such damages are
recoverable by statute or regulation.

10.      WITHHOLDING OF TAXES

         All payments required to be made by the Company to the Employee under
this Agreement shall be subject to the withholding of such amounts relating to
income tax, employment tax and such other taxes and withholdings as the Company
may reasonably determine it should withhold pursuant to any applicable U.S.
Federal, state or local law or regulation.

11.      NOTICE

         All notices, requests and other communications pursuant to this
Agreement shall be in writing and shall be deemed to have been duly given, if
delivered in person or by courier, sent by express, registered or certified
mail, postage prepaid, or sent by facsimile transmission, addressed to the
Employee at the Employee's personal residence as reflected in the Company's
records, and to the Company at 125 Cambridge Park Drive, Cambridge,
Massachusetts 02140. Either party may, by written notice to the other in
accordance herewith, change the address to which notices to such party are to be
delivered or mailed.

12.      GOVERNING LAW

         This Agreement shall be construed, interpreted and enforced in
accordance with the laws of the Commonwealth of Massachusetts, without giving
effect to the choice of law principles thereof.

13.      WAIVER OF BREACH

         Any waiver of any breach of this Agreement shall not be construed to be
a continuing waiver or consent to any subsequent breach on the part either of
the Employee or of the Company. Any waiver to be effective must be in writing
and signed by the Party against whom it is being enforced (either the Employee
or an authorized officer of the Company, as the case may be) and must
specifically refer to the provision of this Agreement which is being waived.

14.      NON-ASSIGNMENT; SUCCESSORS

         This Agreement shall be binding upon and inure to the benefit of the
Parties and their respective successors, heirs (in the case of the Employee) and
assigns. No rights or obligations of the Company under this Agreement may be
assigned or transferred by the Company except that such rights or obligations
may be assigned or transferred pursuant to a merger or

                                       11

<PAGE>

consolidation in which the Company is not the continuing entity, or the sale or
other disposition of all or substantially all of the assets of the Company,
provided that the assignee or transferee is the successor to all or
substantially all of the assets of the Company and such assignee or transferee
assumes the liabilities, obligations and duties of the Company, as contained in
this Agreement, either contractually or as a matter of law. No rights or
obligations of the Employee under this Agreement may be assigned or transferred
by the Employee other than his accrued rights to compensation and benefits,
which may be transferred only by will or operation of law, except as provided in
this Section 14 or in an applicable plan, program, grant or agreement of the
Company or any affiliate. In the event of the Employee's death or a judicial
determination of his incompetence, references in this Agreement to the Employee
shall be deemed, where appropriate, to refer to his beneficiary, estate or other
legal representative.

15.      SEVERABILITY

         If any provision of this Agreement is determined by a court of
competent jurisdiction to be not enforceable in the manner set forth in this
Agreement, the Employee and the Company agree that it is the intention of the
parties that such provision should be enforceable to the maximum extent possible
under applicable law. If any provisions of this Agreement are held to be invalid
or unenforceable, such invalidation or unenforceability shall not affect the
validity or enforceability of any other provision of this Agreement (or any
portion thereof).

16.      ENTIRE AGREEMENT

         This Agreement and the Inventions Agreement constitute the entire
agreement by the Company and the Employee with respect to the subject matter
hereof and except as specifically provided herein, supersedes any and all prior
agreements or understandings between the Employee and the Company with respect
to the subject matter hereof, whether written or oral. Any award agreement
relating to an Equity Award, to the extent inconsistent with any provision of
this Agreement, shall be treated as amended in a manner consistent with the
terms of this Agreement. This Agreement may be amended or modified only by a
written instrument (specifically referencing the provision of this Agreement
being so amended) executed by the Employee and an authorized officer of the
Company.

17.      COUNTERPARTS

         This Agreement may be executed in two or more counterparts.

                                       12

<PAGE>

18.      INDEMNIFICATION

         In the event Employee is named or threatened to be named in any suit or
proceeding arising out of his work for Company within the scope of his
employment hereunder, Employer shall indemnify, defend and hold him harmless, to
the fullest extent permitted by law, from and against any and all damages or
losses arising from such suit, proceeding, or threatened suit or proceeding,
including but not limited to all attorney's fees, litigation costs, verdicts,
judgments or liability.

         IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first written above and this Agreement shall be effective and binding on
the Parties from such date.

                                        IDENIX PHARMACEUTICALS, INC.

                                           /s/ Jean-Pierre Sommadossi
                                        ---------------------------------------
                                        By:     Jean-Pierre Sommadossi, Ph.D.
                                        Title:  Chief Executive OFficer

                                        EMPLOYEE

                                           /s/ David Arkowitz
                                        ---------------------------------------
                                        David Arkowitz.

                                       13<PAGE>
                                                                  EXHIBIT 10.12

                              SEPARATION AGREEMENT

                  This SEPARATION AGREEMENT is dated as of July 16, 2003, and is
entered into by and between Idenix (Massachusetts) Inc., a Massachusetts
corporation (formerly Novirio Pharmaceuticals, Inc.) (the "Company"), and John
P. Dunphy ("Executive").

                  WHEREAS, Executive and the Company are parties to a letter
agreement, dated January 11, 2002 (the "Employment Agreement"), pursuant to
which Executive serves as Chief Financial Officer of the Company; and

                  WHEREAS, in connection with the closing of the transactions
contemplated under that certain Stock Purchase Agreement dated as of March 21,
2003 by and among Novartis Pharma AG, the Company, and certain stockholders of
the Company (the "Purchase Agreement"), the Company underwent a Change in
Control (as defined in the Employment Agreement); and

                  WHEREAS, in accordance with the terms of the Employment
Agreement, following such Change of Control, Executive has tendered his
resignation, which shall be effective as of July 31, 2003 (the "Termination
Date"); and

                  WHEREAS, subject to the terms and conditions contained herein,
Executive and the Company have mutually agreed to embody in this Agreement the
terms and conditions applicable to Executive's resignation of employment with
the Company.

                  NOW, THEREFORE, the parties hereby agree:

         Section 1.        Termination Date. Executive's resignation of
employment with the Company shall be effective on the close of business of the
Termination Date. Executive hereby tenders his resignation from any and all
directorships, committee memberships or any other positions he holds with the
Company or any of its affiliates, effective as of the Termination Date.

         Section 2.        Company Property. On or prior to the Termination
Date, Executive shall return to the Company all Company-owned property in his
possession.

         Section 3.        Termination Benefits.

                  (a)      Accrued Obligations. As soon as practicable following
the Termination Date, but in no event later than the time period required under
applicable law, the Company shall pay Executive all earned but unpaid base
salary through the Termination Date, pay Executive for any accrued but unused
vacation time, and reimburse Executive for any reasonable unreimbursed expenses
incurred in accordance with Company policy.

                  (b)      Company Stock Options. The parties acknowledge that
Executive has vested options (the "Options") to purchase an aggregate of 115,321
shares of common stock, par value $.001 per share (the "Common Stock" of Idenix
Pharmaceuticals, Inc.). Of such aggregate amount, Options to purchase 110,321
shares of Common Stock, exercisable at a price of $3.00 per share, constitute
the remaining part of an award granted on March 28, 2002 and Options to purchase
5,000 shares of Common Stock, exercisable at $8.50 per share, were granted on

<PAGE>

November 20, 2002. Notwithstanding anything contained in the Idenix
Pharmaceuticals, Inc. 1998 Equity Incentive Plan, as amended (the "Equity Plan")
or any stock option agreements evidencing the Options to the contrary, the
Options shall remain exercisable until July 31, 2004. Except as otherwise
provided in this subsection (b), the Options shall remain subject to the terms
and conditions of the Equity Plan and the applicable stock option agreements
evidencing the Options.

                  (c)      Severance Benefits.

                  (i)      On the Termination Date, the Company shall pay
Executive an amount equal to $458,280, which represents the sum of the remainder
of Executive's signing bonus not previously paid, Executive's annual base salary
from the Termination Date through July 31, 2004, and the target bonus Executive
would reasonably have expected to receive for his service to the Company in
2003.

                  (ii)     Provided that Executive elects to continue his health
insurance pursuant to the Consolidated Omnibus Budget Reconciliation Act of
1985, as amended ("COBRA"), the Company shall pay Executive's COBRA premiums
until the earlier of: (A) the first anniversary of the Termination Date, or (B)
the date Executive becomes eligible for health insurance benefits with a new
employer. Unless otherwise notified by Executive in writing, COBRA coverage will
be deemed elected by Executive as of the Termination Date.

         Section 4.        Full Settlement; Compensation and Benefit Plans.
Except as provided in the last sentence of this Section 4, the amounts payable
under Section 3 above shall constitute full settlement and satisfaction with
respect to all obligations and liabilities of the Company and its respective
affiliates, officers, directors, trustees, employees, shareholders,
representatives and/or agents to Executive. Except as otherwise specifically
provided in this Agreement, by law or pursuant to the express provisions of any
Company employee benefit plan, Executive's participation in all employee benefit
plans and executive compensation plans and practices of the Company shall
terminate on the Termination Date. Notwithstanding anything in this Section 4 to
the contrary, nothing in this Agreement shall affect Executive's right to (i)
indemnification under the terms of the Company's charter and bylaws, (ii)
benefits under any directors' and officers' liability policy maintained by the
Company, in accordance with its terms, and (iii) benefits and any subsequent
payments to which Executive is or becomes entitled as a result of Executive's
sale pursuant to the Purchase Agreement of certain shares of Common Stock
previously owned by Executive..

         Section 5.        Release of Claims. (a) On the Termination Date,
Executive shall execute a General Release of Claims in the form attached hereto
as Exhibit A (the "Executive Release") and the obligations of the Company shall
become effective if the Executive has not revoked the Executive Release at the
expiration of seven day revocation period contemplated therein.

                  (b)      The Company on behalf of itself and its affiliates
hereby represents that as of the date of this Agreement, it does not have any
knowledge of actual, pending or threatened claims against the Executive by the
Company, or any of its affiliates, shareholders, employees or directors of any
kind whatsoever, nor does it have knowledge of facts or circumstances that could
reasonably be expected to give rise to any such claims.

                                       2
<PAGE>

         Section 6.        Taxes. The payments due to Executive under this
Agreement shall be subject to reduction to satisfy all applicable Federal, state
and local withholding tax obligations.

         Section 7.        Non-Admission. Executive and the Company expressly
acknowledge that this Agreement does not constitute an admission by the other
party of any violation of any employment law, regulation, ordinance, or
administrative procedure, or any other federal, state, or local law, common law,
regulation or ordinance, liability for which is expressly denied.

         Section 8.        Non-Disparagement. Executive hereby agrees that,
except as required by applicable law, or compelled by process of law, at any
time following the date hereof, he shall not hereafter make any false,
derogatory or disparaging statements about the Company, which in the case of the
Company shall include its subsidiaries, affiliates and any of the Company's
current officers, directors, employees, or shareholders, or about the Company's
financial condition or business affairs. The Company agrees that, except as
required by applicable law, or compelled by process of law, at any time
following the date hereof, neither it nor any of its affiliates will make any
false, derogatory or disparaging statements about Executive. Additionally, the
Company will direct: (1) those employees of the Company or its affiliates who
have knowledge of the circumstances giving rise to his separation and the terms
of this Agreement and (2) its officers and directors, not make any false,
derogatory or disparaging statements about Executive.

         Section 9.        Non Disclosure and Assignment of Inventions.
Executive acknowledges and reaffirms his obligation to keep confidential all
non-public information concerning the Company, its business, affairs, plans or
prospects (the "Confidential Information") which the Executive acquired during
the course of his employment with the Company and to assign to the Company all
inventions and discoveries made during the course of his employment or
subsequent thereto if such inventions or discoveries were made using or in any
way relying upon the Confidential Information, as stated more fully in the
Invention and Non-Disclosure Agreement_which Executive executed at the inception
of his employment which remains in full force and effect.

         Section 10.       Confidentiality of Agreement. To the extent permitted
by law, Executive and the Company hereby agree to keep confidential the terms
and content of this Agreement and the contents of the negotiations and
discussions resulting in this Agreement, and none of the above shall be
disclosed except as required by applicable law, regulation or order of a court
or governmental agency, to either party's counsel and/or financial advisors, or
as otherwise agreed in writing by the parties' authorized agent, except that
Executive may disclose the above to his immediate family or any potential future
employers of Executive, and the Company may disclose the above to its directors,
officer and employees with a legitimate business need to know. Notwithstanding
anything to the contrary in this Agreement or any other document to which
Executive is a party to or otherwise furnished to Executive, each of Executive
and the Company and each of its directors, employees, representatives or agents,
as applicable, may disclose to any and all persons, without limitation of any
kind, the "tax treatment" and "tax structure" of the arrangements contained
herein together with any and all materials relating to such tax treatment or tax
structure; provided, however, that the foregoing authorization does not permit
disclosure of the identities of the parties hereto. For this purpose, the "tax
treatment" is the purported or claimed Federal income tax treatment of the
arrangement and the "tax structure" is any fact that

                                       3
<PAGE>

may be relevant to understanding the purported or claimed Federal income tax
treatment of the arrangement.

         Section 11.       Opportunity for Advice. By signing this Agreement,
Executive acknowledges that with the advice of the Company, he has had a
reasonable opportunity to consider advice from his legal counsel, and that, he
in fact consulted with an attorney prior to executing this Agreement and the
General Release of Claims attached as Exhibit A fully understanding the terms of
this Agreement, Executive is entering into this Agreement knowingly and
voluntarily.

         Section 12.       Entire Agreement. This Agreement and the General
Release of Claims attached hereto as Exhibit A represent the entire agreement of
the parties with respect to Executive's employment and termination thereof.
Except as specifically provided herein, this Agreement shall supersede the
Employment Agreement in all respects effective as of the Termination Date.
Nothing in this Section 12, however, shall modify, cancel or supersede the
obligations under the Invention and Non-Disclosure Agreement executed by
Executive on March 18, 2002, and the obligations of Executive arising pursuant
to the Purchase Agreement and other agreements related thereto to which the
Executive is a party.

         Section 13.       Applicable Law. THIS AGREEMENT SHALL BE GOVERNED BY
AND CONSTRUED AND ENFORCED IN ACCORDANCE WITH THE LAWS OF THE COMMONWEALTH OF
MASSACHUSETTS APPLICABLE TO CONTRACTS MADE AND TO BE PERFORMED ENTIRELY WITHIN
SUCH STATE. EXECUTIVE HEREBY IRREVOCABLY SUBMITS TO AND ACKNOWLEDGES AND
RECOGNIZES THE JURISDICTION OF THE COURTS IN THE COMMONWEALTH OF MASSACHUSETTS,
OR, IF APPROPRIATE, A FEDERAL COURT IN MASSACHUSETTS OVER ANY SUIT, ACTION OR
OTHER PROCEEDING ARISING OUT OF, UNDER OR IN CONNECTION WITH THIS AGREEMENT OR
THE SUBJECT MATTER HEREOF.

         Section 14.       Severability. In the event that any provision or
portion of this Agreement shall be determined to be invalid or unenforceable for
any reason, in whole or in part, the remaining provisions of this Agreement
shall be unaffected thereby and shall remain in full force and effect to the
fullest extent permitted by law.

         Section 15.       Counterparts. This Agreement may be executed in one
or more counterparts, each of which shall be deemed an original and all of which
together shall be considered one and the same agreement.

                                    [Signatures appear on following page.]

                                       4
<PAGE>

                  IN WITNESS WHEREOF, the parties have executed this Agreement
as of the date and year first above written.

                                        /s/ John P. Dunphy
                                        ---------------------------------
                                        John P. Dunphy

                                        IDENIX (MASSACHUSETTS) INC.

                                        By: /s/ Jean-Pierre Sommadossi
                                        ---------------------------------
                                           Name: Jean-Pierre Sommadossi
                                           Title: Chief Executive Officer

                                       5
<PAGE>

                            GENERAL RELEASE OF CLAIMS

                  1.       John P. Dunphy ("Executive"), for himself and his
family, heirs, executors, administrators, legal representatives and their
respective successors and assigns, in exchange for the consideration contained
in the Separation Agreement to which this release is attached as Exhibit A (the
"Separation Agreement"), which Executive acknowledges is in addition to any
amounts to which he would have otherwise been entitled but for the Separation
Agreement and execution of this General Release of Claims, does hereby release
and forever discharge Idenix Massachusetts, Inc. (the "Company") and any of its
subsidiaries, parent or affiliated companies, and their current or former
directors, officers, employees, shareholders, agents or plan administrators
(each in their individual and corporate capacities) (together with the Company,
the "Released Parties") from any and all actions, causes of action, suits,
controversies, claims, demands, obligations, liabilities and expenses (including
attorney's fees and costs) whatsoever, for or by reason of any matter, cause or
thing whatsoever, whether known or unknown including, but not limited to, all
employment discrimination claims under Title VII of the Civil Rights Act of
1964, 42 U.S.C. Section 2000e et seq., the Age Discrimination in Employment Act,
29 U.S.C. Section 621 et seq., the Americans With Disabilities Act of 1990, 42
U.S.C. Section 12101 et seq., and the Massachusetts Fair Employment Practices
Act, M.G.L. c.151B, Section 1 et seq., all as amended; all claims arising out of
the Family and Medical Leave Act, 29 U.S.C. Section 2601 et seq., the Fair
Credit Reporting Act, 15 U.S.C. Section 1681 et seq., the Employee Retirement
Income Security Act of 1974 ("ERISA"), 29 U.S.C. Section 1001 et M., the
Massachusetts Civil Rights Act, M.G.L. c.12 Sections 11H and 111, the
Massachusetts Equal Rights Act, M.G.L. c.93, Section 102 and M.G.L. c.214,
Section 1C, the Massachusetts Labor and Industries Act, M.G.L. c.149, Section 1
et seq., the Massachusetts Privacy Act, M.G.L. c. 214, Section 1B, and the
Massachusetts Maternity Leave Act Act, M.G.L. c. 149, Section 105(d), all as
amended; all common law claims including, but not limited to, actions in tort,
defamation and breach of contract (including all claims relating to or arising
under the offer letter of employment dated January 11, 2002), wrongful
discharge, or intentional infliction of emotional distress; all claims to any
non-vested ownership interest in the Company, contractual or otherwise,
including but not limited to claims to stock or stock options; and any claim or
damage arising out of Executive's employment with or separation from the Company
(including a claim for retaliation) under any common law theory or any federal,
state or local statute or ordinance not expressly referenced above. Executive
acknowledges that the Company encouraged him to consult with an attorney of his
choosing, and through this General Release of Claims encourages him to consult
with his attorney with respect to possible claims under the Age Discrimination
in Employment Act ("ADEA") and that he understands that the ADEA is a Federal
statute that, among other things, prohibits discrimination on the basis of age
in employment and employee benefits and benefit plans. Without limiting the
generality of the release provided above, Executive expressly waives any and all
claims under ADEA that he may have as of the date hereof. Executive further
understand that by signing this General Release of Claims he is in fact waiving,
releasing and forever giving up any claim under the ADEA as well as all other
laws within the scope of this paragraph 1 that may have existed on or prior to
the date hereof. Notwithstanding anything in this paragraph 1 to the contrary,
this General Release of Claims shall not apply to (i) any actions to enforce
rights arising under, or any claim for benefits which may be due Executive
pursuant to, the Separation Agreement, (ii) any rights or claims that may arise
as a result of events occurring after the date this General Release of Claims is
executed, (iii) any indemnification rights Executive may have as a former
officer or director of the Company, (iv) any claims for benefits under any
directors' and officers' liability policy

                                      A-1-
<PAGE>

maintained by the Company, in accordance with its terms, and (v) any payments
and benefits Executive is entitled to as a result of the sale of his shares in
the Company pursuant to the Purchase Agreement (as defined in the Separation
Agreement).

                  2.       Notwithstanding paragraph 1 above, nothing in this
General Release of Claims prevents Executive from filing, cooperating with, or
participating in any proceeding before the Equal Employment Opportunity
Commission or a state fair employment practices agency (except that Executive
acknowledges that he may not be able to recover any monetary benefits in
connection with any such claim, charge or proceeding).

                  3.       Executive hereby acknowledges that the Company has
informed him that he has up to twenty-one (21) days to sign this General Release
of Claims and he may knowingly and voluntarily waive that twenty-one (21) day
period by signing this General Release of Claims earlier. Executive also
understands that he shall have seven (7) days following the date on which he
signs this General Release of Claims within which to revoke it by providing a
written notice of his revocation to the Company.

                  4.       Executive acknowledges that this General Release of
Claims will be governed by and construed and enforced in accordance with the
internal laws of the Commonwealth of Massachusetts applicable to contracts made
and to be performed entirely within such State.

                  5.       Executive acknowledges that he has read this General
Release of Claims, that he has been advised that he should consult with an
attorney before he executes this general release of claims, and that he
understands all of its terms and executes it voluntarily and with full knowledge
of its significance and the consequences thereof.

                  6.       This General Release of Claims shall take effect on
the eighth day following Executive's execution of this General Release of Claims
unless Executive's written revocation is delivered to the Company within seven
(7) days after such execution.

                                            /s/ John P. Dunphy
                                            ---------------------------
                                            John P. Dunphy
                                            July 31, 2004

                                      A-2-

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