**Exhibit 31.1**

**CERTIFICATIONOF THE CHIEF EXECUTIVE OFFICER PURSUANT TO SECTION 302 OF THE SARBANES-OXLEYACT OF 2002**

I, Christoph Westphal,certify that:

1. I have reviewed thisquarterly report on Form 10-Q of Sirtris Pharmaceuticals, Inc.;

2. Based on my knowledge,this quarterly report does not contain any untrue statement of a material factor omit to state a material fact necessary to make the statements made, inlight of the circumstances under which such statements were made, notmisleading with respect to the period covered by this report;

3. Based on my knowledge,the financial statements, and other financial information included in thisquarterly report, fairly present in all material respects the financialcondition, results of operations and cash flows of the registrant as of, andfor, the periods presented in this report;

4. The registrant’s othercertifying officers and I are responsible for establishing and maintainingdisclosure controls and procedures (as defined in Exchange ActRules 13a-15(e) and 15d-15(e)) and internal control over financialreporting (as defined in Exchange Act Rules 13a-15(f) and15d-15(f))  for the registrant and have:

a. Designed such disclosure controls and procedures, or caused suchdisclosure controls and procedures to be designed under our supervision, toensure that material information relating to the registrant, including itsconsolidated subsidiaries, is made known to us by others within those entities,particularly during the period in which this report is being prepared;

b. Evaluated the effectiveness of the registrant’s disclosure controlsand procedures and presented in this report our conclusions about theeffectiveness of the disclosure controls and procedures, as of the end of theperiod covered by this report based on such evaluation; and

c. Disclosed in this report any change in the registrant’s internalcontrol over financial reporting that occurred during the registrant’s mostrecent fiscal quarter (the registrant’s fourth fiscal quarter in the case of anannual report) that has materially affected, or is reasonably likely tomaterially affect, the registrant’s internal control over financial reporting;and

5. The registrant’s othercertifying officers and I have disclosed, based on our most recent evaluationof internal control over financial reporting, to the registrant’s auditors andthe audit committee of registrant’s board of directors (or persons performingthe equivalent functions):

a. All significant deficiencies and material weaknesses in the designor operation of internal control over financial reporting which are reasonablylikely to adversely affect the registrant’s ability to record, process,summarize and report financial information; and

b. Any fraud, whether or not material, that involves management orother employees who have a significant role in the registrant’s internalcontrol over financial reporting.

Date: August 13, 2007
| /s/ Christoph Westphal                                                |   |
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| Christoph  Westphal                                                   |
| President and  Chief Executive Officer  (principal executive officer) |

 

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