<PAGE>

                                                                    Exhibit 10.5

                                 ARCHEMIX CORP.
                               1 Hampshire Street
                               Cambridge, MA 02139

August 24, 2004

Mr. Page Bouchard, D.V.M., D.A.C.V.P.
12 Rock O'Dundee Road
Andover, MA 01810

Dear Page:

     I am very pleased to provide you with a summary of the terms and conditions
of your anticipated employment by Archemix Corp. ("Archemix"). The following
sets forth the proposed terms and conditions of your offer of employment. We
hope that you choose to join the Archemix team and look forward to a mutually
beneficial relationship.

1. POSITION Your position will be Sr. Vice President of Preclinical Drug
Discovery and Development, reporting to Dr. Errol De Souza, President and Chief
Executive Officer. As you progress with Archemix, your position and assignments
are, of course, subject to change. As an Archemix employee, we expect that you
will perform any and all duties and responsibilities normally associated with
your position in a satisfactory manner and to the best of your abilities at all
times. Your performance will be reviewed formally sometime around January, 2005,
and on a periodic basis thereafter as long as you remain employed by Archemix.

2. JOB DESCRIPTION As Sr. Vice President of Preclinical Drug Discovery and
Development, you will have the following primary responsibilities: Develop and
implement a strategy and coordinate all internal and external efforts related to
three major focus areas within Research and Development which include IN VIVO
PHARMACOLOGY, DMPK AND DRUG SAFETY EVALUATION. In this capacity you will be
required to interface effectively with the Aptamer Discovery group to facilitate
effective transition of compounds from Lead to Candidate stage and with the
Clinical Development group including Regulatory to transition Candidates through
Preclinical Development and Human testing. In your capacity as a member of the
senior management team of the Company, you will have an opportunity to provide
input and guidance on all major aspects of the Company.

3. STARTING DATE/NATURE OF RELATIONSHIP If you accept this offer, your
employment with Archemix will begin on September 13th, 2004. You should report
to Archemix on that date, and you will be expected to devote all of your working
time to the performance of your duties at Archemix throughout your employment
with Archemix. No provision of this letter shall be construed to create an
express or implied employment contract, or a promise of employment for

<PAGE>

any specific period of time. Your employment with Archemix is at-will
employment, which may be terminated by you or Archemix at any time for any
reason.

4. COMPENSATION AND BENEFITS Your initial base pay shall be at a gross rate of
$20,000 per month, minus customary deductions for federal and state taxes plus
any elective benefits or deductions. You may be eligible for an annual bonus
target equal to 25% of your base salary, based on overall company and individual
performance. Potential bonus amounts may be compensated in the form of equity or
cash or a combination of each. Subject to approval by the Board of Directors (or
an appropriate Committee appointed by the Board of Directors), and pursuant to a
written stock option agreement entered into by and between you and Archemix (the
"Option Agreement") pursuant to the Corporation's 2001 Employee, Director and
Consultant Stock Plan (the "2001 Plan"), as amended, Archemix will grant you an
option to purchase 400,000 shares of Archemix Common Stock (the "Option"). The
exercise price of the Option will be the fair market value of the Corporation's
Common Stock on the date the Option is granted and such Option shall be subject
to the terms and conditions of the Option as set forth in the 2001 Plan and the
Option Agreement. To the extent permissible under applicable federal tax law,
the Option will be an incentive stock option with the Option vesting on the
following schedule: twenty-five percent (25%) of such shares shall vest on the
first (1st) anniversary of the date of your option agreement; after the first
(1st) anniversary, the remaining seventy-five percent (75%) of such shares shall
vest ratably in three (3) month periods until the fourth (4th) anniversary your
option agreement. Such vesting shall commence on the first anniversary September
13th, 2004 and quarterly thereafter.

     In addition to your compensation, you may take advantage of various
benefits offered by Archemix, including group medical plans and paid vacation
time. Currently, Archemix provides fifteen vacation days per year, accruable at
the rate of 1 1/4 days per month. In addition Archemix recognizes 8 scheduled
and 3 floating holidays per calendar year. These benefits, of course, may be
modified or changed from time to time at the sole discretion of Archemix, and
the provision of such benefits to you in no way changes or impacts your status
as an at-will employee. Archemix's present benefit structure and other important
information about the benefits for which you may be eligible will be described
to you upon the commencement of your employment. Where a particular benefit is
subject to a formal plan (for example, medical insurance or life insurance),
eligibility to participate in and receive any particular benefit is governed
solely by the applicable plan document. Should you ever have any questions about
Archemix benefits, you should ask the Human Resources Director for a copy of the
applicable plan document.

5. BEFORE YOU START Your employment with Archemix is conditioned on your
eligibility to work in the United States. On your first day, you must complete
an I-9 Form and provide Archemix with any of the accepted forms of
identification specified on the I-9 Form.

6. CONFIDENTIALITY Archemix considers the protection of its confidential
information, proprietary materials and goodwill to be extremely important. Given
the confidential nature of various aspects of our business, you may not discuss
the fact or terms of this offer or any employment discussions with anyone other
than a member of the Archemix management team and members of your immediate
family (and, if relevant, your recruiting firm, financial advisor


                                       2

<PAGE>

or lawyer). In addition, as is true of all Archemix employees, you will be
required to sign an agreement relating to confidentiality, non-competition,
non-solicitation and work product on your first day of work as a condition of
this offer of employment.

7. MISCELLANEOUS This letter constitutes our entire offer regarding the terms
and conditions of your prospective employment with Archemix. It supersedes any
prior agreements, or other promises or statements (whether oral or written)
regarding the offered terms of employment. The terms of your employment shall be
governed by the law of the Commonwealth of Massachusetts. By accepting this
offer of employment, you agree that any action, demand, claim or counterclaim in
connection with any aspect of your employment with Archemix, or any separation
of employment (whether voluntary or involuntary) from Archemix, shall be
resolved by a judge alone, and you waive and forever renounce your right to a
trial before a civil jury.

     You may accept this offer of employment and the terms and conditions hereof
by signing the enclosed additional copy of this letter. Your signature on the
copy of this letter and your submission of the signed copy to me will evidence
your agreement with the terms and conditions set forth herein. This offer will
expire on SEPTEMBER 1ST, 2004, unless accepted by you prior to such date.

     We are excited to offer you the opportunity to join Archemix, and we look
forward to having you aboard. We are pleased that you have chosen to join the
Archemix team, and confident that you will make an important contribution to our
unique and exciting enterprise.

                                        Sincerely,


                                        Errol De Souza
                                        ----------------------------------------
                                        President and Chief Executive Officer

Acknowledgment:

     I ____________________________ have read, understand, and accept employment
on the terms and conditions outlined in this letter. I am not relying on any
representations made to me by anyone other than as set forth above.


-------------------------------------
Signature

-------------------------------------
Date


                                        3