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                                                                   Exhibit 10.47

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MOMENTA PHARMACEUTICALS, INC.             675 WEST KENDALL STREET               T: 617.491.9700 F: 617.621.0431
                                          CAMBRIDGE, MA 02142                   WWW.MOMENTAPHARMA.COM
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[MOMENTA LOGO]


                                November 2, 2004

Joseph Tyler
22 Forster Street
Somerville, Massachusetts 02145

Dear Joe:

In connection with the termination of your employment with Momenta
Pharmaceuticals, Inc. (the "Company") effective as of November 15, 2004 (the
"Termination Date"), in particular from your position of Vice President of
Manufacturing, you are eligible to receive the benefits described in this
letter.

By signing and returning this letter, you will be entering into a binding
agreement with the Company and will be agreeing to the terms and conditions set
forth in the numbered paragraphs below, including the release of claims set
forth in paragraph 4. Therefore, you are advised to consult with an attorney
before signing this letter and you may take up to twenty-one (21) days to do so.
If you sign this letter, you may change your mind and revoke your agreement
during the seven (7) day period after you have signed it. If you do not so
revoke, this letter will become a binding agreement between you and the Company
after the expiration of the seven (7) day revocation period.

Regardless of whether you sign this letter you will receive payment upon your
termination for any unused vacation time accrued through the Termination Date
and you may elect to continue receiving group medical insurance pursuant to the
federal "COBRA" law, 29 U.S.C. Section 1161 ET SEQ. All premium costs shall be
paid by you on a monthly basis for as long as, and to the extent that, you
remain eligible for COBRA continuation. You should consult the COBRA materials
to be provided by the Company for details regarding these benefits. All other
benefits, including life insurance and long term disability, will cease upon
your Termination Date. Pursuant to the Company's Amended and Restated 2002 Stock
Incentive Plan, you will have up to thirty (30) days after the Termination Date
to exercise any previously vested stock rights you may have (the "Exercise
Period") unless extended as set forth below. All unvested stock rights will be
cancelled on the Termination Date.

If, after reviewing this letter agreement with an attorney, you find the terms
and conditions are satisfactory to you, you should sign and return this letter
to Richard P. Shea in the enclosed envelope by November 23, 2004.

The following numbered paragraphs set forth the terms and conditions which will
apply if and only if you timely sign and return this letter agreement and do not
revoke it within the seven (7) day period:

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1.    TERMINATION DATE - Your effective date of termination from the Company is
      November 15, 2004 (the "Termination Date").

2.    OPTION ACCELERATION - In consideration for the release of claims set forth
      in Paragraph 4 and your agreeing that you will use your best efforts to
      facilitate the effective transition of your departure and the integration
      of your replacement, the Company has agreed to a one-year acceleration of
      the vesting of your stock options that are unvested as of the Termination
      Date. In addition, subject to the Company's Chief Executive Officer's
      assessment of your performance during such transition period and for a
      reasonable time thereafter, at the Company's sole discretion, you may be
      entitled to additional acceleration of the vesting of your stock options
      unvested as of the Termination Date, however such acceleration shall not
      in any event exceed an additional one-year acceleration. If applicable,
      you will be notified of such additional accelerated options in writing by
      the earlier of the 8th day after the execution of this letter agreement or
      December 1, 2004.

3.    EXTENSION TO EXERCISE PERIOD - As further consideration for the release of
      claims set forth in Paragraph 4 and your agreeing to use your best efforts
      to facilitate the effective transition of your departure and the
      integration of your replacement, the Company has agreed to amend the
      Exercise Period to provide you with a ninety (90) day exercise period (the
      "Extended Exercise Period") to exercise both previously vested and any
      accelerated options.

4.    RELEASE - In consideration of the consideration set forth herein, which
      you acknowledge you would not otherwise be entitled to receive, you hereby
      fully, forever, irrevocably and unconditionally release, remise and
      discharge the Company, its officers, directors, stockholders, corporate
      affiliates, subsidiaries, parent companies, agents and employees (each in
      their individual and corporate capacities) (hereinafter, the "Released
      Parties") from any and all claims, charges, complaints, demands, actions,
      causes of action, suits, rights, debts, sums of money, costs, accounts,
      reckonings, covenants, contracts, agreements, promises, doings, omissions,
      damages, executions, obligations, liabilities, and expenses (including
      attorneys' fees and costs), of every kind and nature which you ever had or
      now have against the Released Parties arising out of your employment with
      and/or separation from the Company, 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., the Family and
      Medical Leave Act, 29 U.S.C. Section 2601 ET SEQ., the Worker Adjustment
      and Retraining Notification Act ("WARN"), 29 U.S.C. Section 2101 ET SEQ.,
      and the Rehabilitation Act of 1973, 29 U.S.C. Section 701 ET SEQ., all as
      amended; all claims arising out of 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 seq., the Massachusetts Fair
      Employment Practices Act., M.G.L. c.151B, Section 1 ET SEQ., the
      Massachusetts Civil Rights Act, M.G.L. c.12 Sections 11H and 11I, 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

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      Massachusetts Privacy Act, M.G.L. c. 214, Section 1B, and the
      Massachusetts Maternity Leave 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; 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 except as provided in
      Paragraphs 2 and 3; and any claim or damage arising out of your 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; provided, however, that nothing
      in this letter agreement prevents you from filing, cooperating with, or
      participating in any proceeding before the EEOC or a state Fair Employment
      Practices Agency (except that you acknowledge that you may not be able to
      recover any monetary benefits in connection with any such claim, charge or
      proceeding or from enforcing the terms of this letter agreement).

5.    NON-DISCLOSURE, NON-COMPETITION AND NON-SOLICITATION OBLIGATIONS - You
      acknowledge and reaffirm your obligation to keep confidential and not to
      disclose any and all non-public information concerning the Company which
      you acquired during the course of your employment with the Company,
      including, but not limited to, any non-public information concerning the
      Company's business affairs, business prospects and financial condition as
      is stated more fully in the Employee Nondisclosure, Noncompetition and
      Assignment of Intellectual Property Agreement you executed at the
      inception of your employment which remains in full force and effect. You
      further acknowledge and reaffirm your obligations under the Employee
      Nondisclosure, Noncompetition and Assignment of Intellectual Property
      Agreement you previously executed for the benefit of the Company at the
      inception of your employment, which also remain(s) in full force and
      effect, a copy of which is attached hereto for your convenience.

6.    RETURN OF COMPANY PROPERTY - You confirm that you have returned to the
      Company all keys, files, records (and copies thereof), equipment
      (including, but not limited to, computer hardware, software and printers,
      wireless handheld devices, cellular phones, pagers, etc.), parking passes,
      Company identification, Company vehicles and any other Company-owned
      property in your possession or control and have left intact all electronic
      Company documents, including but not limited to those which you developed
      or help develop during your employment. You further confirm that you have
      cancelled all accounts for your benefit, if any, in the Company's name,
      including but not limited to, credit cards, telephone charge cards,
      cellular phone and/or pager accounts and computer accounts.

7.    BUSINESS EXPENSES AND COMPENSATION - You acknowledge that you have been
      reimbursed by the Company for all business expenses incurred in
      conjunction with the performance of your employment and that no other
      reimbursements are owed to you. You further acknowledge that you have
      received payment in full for all services rendered in conjunction with
      your employment by the Company and that no other compensation is owed to
      you.

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8.    NON-DISPARAGEMENT - You understand and agree that you shall not make any
      false, disparaging or derogatory statements to any media outlet, industry
      group, financial institution or current or former employee, consultant,
      vendor, client or customer of the Company regarding the Company or any of
      its directors, officers, employees, agents, representatives or about the
      Company's business affairs and financial condition. The Company agrees
      that it will direct its officers and managers with knowledge of this
      letter agreement and Company's Senior Directors not to make any
      disparaging or derogatory statements about you. The Company will also
      direct its officers and managers with knowledge of this letter agreement
      and Company's Senior Directors to respond to any inquiries from
      prospective employers by confirming your title and dates of employment
      only.

9.    COOPERATION WITH CLAIMS OR ACTIONS - You agree to make yourself available
      upon receiving reasonable notice to cooperate with the Company in the
      defense or prosecution of any claims or actions which may be brought
      against or on behalf of the Company. You agree that you will notify the
      Company promptly in the event that you are served with a subpoena or are
      asked to provide a third party with information concerning any actual or
      potential complaint or claim against the Company or being brought by the
      Company. You agree that, if called to testify in any proceeding in court,
      at arbitration or before an administrative agency, you will testify
      truthfully under all circumstances. Subject to the foregoing, you agree to
      make yourself available upon receiving reasonable notice to meet with
      counsel for the Company in connection with the Company's preparation of
      its claims or defenses.

10.   AMENDMENT - This letter agreement shall be binding upon the parties and
      may not be modified in any manner, except by an instrument in writing of
      concurrent or subsequent date signed by duly authorized representatives of
      the parties hereto. This letter agreement is binding upon and shall inure
      to the benefit of the parties and their respective agents, assigns, heirs,
      executors, successors and administrators.

11.   WAIVER OF RIGHTS - No delay or omission by the Company in exercising any
      right under this letter agreement shall operate as a waiver of that or any
      other right. A waiver or consent given by the Company on any one occasion
      shall be effective only in that instance and shall not be construed as a
      bar or waiver of any right on any other occasion.

12.   VALIDITY - Should any provision of this letter agreement be declared or be
      determined by any court of competent jurisdiction to be illegal or
      invalid, the validity of the remaining parts, terms or provisions shall
      not be affected thereby and said illegal or invalid part, term or
      provision shall be deemed not to be a part of this letter agreement.

13.   CONFIDENTIALITY - You understand and agree that as a condition of the
      stock option provisions set forth in paragraphs 2 and 3 and of the terms
      and conditions herein described, the terms and contents of this letter
      agreement, and the contents

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      of the negotiations and discussions resulting in this letter agreement,
      shall be maintained as confidential by you and your agents and
      representatives and shall not be disclosed except to the extent required
      by federal or state law or as otherwise agreed to in writing by the
      Company. You further understand that Company may be required to file this
      letter agreement with the Securities Exchange Commission and/or other
      governmental agencies.

14.   NATURE OF AGREEMENT - You understand and agree that this letter agreement
      is a separation agreement and does not constitute an admission of
      liability or wrongdoing on the part of the Company.

15.   ACKNOWLEDGMENTS - You acknowledge that you have been given at least
      twenty-one (21) days to consider this letter agreement and that the
      Company advised you to consult with an attorney of your own choosing prior
      to signing this letter agreement. You understand that you may revoke this
      letter agreement for a period of seven (7) days after you sign this letter
      agreement, and the letter agreement shall not be effective or enforceable
      until the expiration of this seven (7) day revocation period. YOU
      UNDERSTAND AND AGREE THAT BY ENTERING INTO THIS LETTER AGREEMENT YOU ARE
      WAIVING ANY AND ALL RIGHTS OR CLAIMS YOU MIGHT HAVE UNDER THE AGE
      DISCRIMINATION IN EMPLOYMENT ACT, AS AMENDED BY THE OLDER WORKERS BENEFIT
      PROTECTION ACT, AND THAT YOU HAVE RECEIVED CONSIDERATION BEYOND THAT TO
      WHICH YOU WERE PREVIOUSLY ENTITLED.

16.   VOLUNTARY ASSENT - You affirm that no other promises or agreements of any
      kind have been made to or with you by any person or entity whatsoever to
      cause you to sign this letter agreement, and that you fully understand the
      meaning and intent of this letter agreement. You state and represent that
      you have had an opportunity to fully discuss and review the terms of this
      letter agreement with an attorney. You further state and represent that
      you have carefully read this letter agreement understand the contents
      herein, freely and voluntarily assent to all of the terms and conditions
      hereof, and sign your name of your own free act.

17.   APPLICABLE LAW - This letter agreement shall be interpreted and construed
      by the laws of the Commonwealth of Massachusetts, without regard to
      conflict of laws provisions. You hereby irrevocably submit to and
      acknowledge and recognize the jurisdiction of the courts of the
      Commonwealth of Massachusetts, or if appropriate, a federal court located
      in Massachusetts (which courts, for purposes of this letter agreement, are
      the only courts of competent jurisdiction), over any suit, action or other
      proceeding arising out of, under or in connection with this letter
      agreement or the subject matter hereof.

18.   ENTIRE AGREEMENT - This letter agreement contains and constitutes the
      entire understanding and agreement between the parties hereto with respect
      to your termination of employment and the settlement of claims against the
      Company and cancels all previous oral and written negotiations,
      agreements, commitments and writings in connection therewith. Nothing in
      this paragraph, however, shall modify, cancel or supersede your
      obligations set forth in paragraph 5 herein.

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If you have any questions about the matters covered in this letter, please call
me.


                                        Very truly yours,

                                        Momenta Pharmaceuticals, Inc.

                                        By: /s/ Alan L. Crane
                                            -----------------

                                        Name:  Alan L. Crane

                                        Title: Chief Executive Officer

I hereby agree to the terms and conditions set forth above. I have been given at
least twenty-one (21) days to consider this agreement and I have chosen to
execute this on the date below. I intend that this letter agreement become a
binding agreement between me and the Company if I do not revoke my acceptance in
seven (7) days.


/s/ Joseph Tyler                        Date: November 15, 2004
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Employee Name:


To be returned by November 23, 2004.

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