Filed by Aon Corporation

Pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-6(j)

under the Securities Exchange Act of 1934

Subject Company: Hewitt Associates, Inc.

Commission File No.: 001-31351

Commission File No. for Registration Statement

on Form S-4: 333-168320

This correspondence was mailed to Aon stockholders on or about September 1, 2010.

AON CORPORATION

200 East Randolph Street

Chicago, Illinois 60601

August 31, 2010

Dear Stockholder:

We have previously sent to you proxy material for the Special Meeting of Aon Corporation stockholders, to be held on September 20, 2010.  Your Board of Directors recommends that stockholders vote FOR Aon’s acquisition of Hewitt Associates, Inc.

Your vote is important, no matter how many or how few shares you may own.  If you have not already done so, please vote TODAY by telephone, via the Internet, or by signing, dating and returning the enclosed proxy card in the envelope provided.

Very truly yours,

Gregory C. Case

President and Chief Executive Officer

REMEMBER:

You can vote your shares by telephone, or via the Internet.

Please follow the easy instructions on the enclosed card.

If you have any questions, or need assistance in voting

your shares, please call our proxy solicitor,

INNISFREE M&A INCORPORATED

TOLL-FREE, at (877) 456-3463.

Additional Information

This communication does not constitute an offer to sell or the solicitation of an offer to buy our securities or the solicitation of any vote or approval.  This communication is being made in respect of the proposed transaction involving Aon Corporation (“Aon”) and Hewitt Associates, Inc. (“Hewitt”).  In connection with the proposed merger, Aon filed with the SEC a definitive joint proxy statement, which also constitutes a prospectus of Aon.  The joint proxy statement/prospectus was mailed to Aon stockholders and Hewitt stockholders on or about August 19, 2010. Before making any voting or investment decision, investors and stockholders are urged to read carefully in their entirety the definitive joint proxy statement/prospectus regarding the proposed transaction and any other relevant documents filed by either Aon or Hewitt with the SEC when they become available because they contain and will contain important information about the proposed transaction.  You may obtain copies of all documents filed with the SEC regarding this transaction, free of charge, at the SEC’s website (www.sec.gov), by accessing Aon’s website at www.aon.com under the heading “Investor Relations” and then under the link “SEC Filings” and from Aon by directing a request to Aon at Aon Corporation, 200 E. Randolph Street, Chicago, Illinois 60601, Attention: Investor Relations, and by accessing Hewitt’s website at www.hewitt.com under the heading “Investor Relations” and then under the link “Reports & SEC Filings” and from Hewitt by directing a request to Hewitt at Hewitt Associates, Inc., 100 Half Day Road, Lincolnshire, Illinois 60069, Attention: Investor Relations.

Aon and Hewitt and their respective directors and executive officers and certain other members of management and employees may be deemed to be participants in the solicitation of proxies in respect of the proposed transaction. You can find information about Aon’s directors and executive officers in its definitive proxy statement filed with the SEC on April 7, 2010. You can find information about Hewitt’s directors and executive officers in its definitive proxy statement filed with the SEC on December 16, 2009. Other information regarding the participants in the proxy solicitation and a description of their direct and indirect interests, by security holdings or otherwise, are contained in the definitive joint proxy statement/prospectus filed by Aon with the SEC and will be contained in other relevant materials to be filed by Aon or Hewitt with the SEC when they become available. You can obtain free copies of these documents from Aon and Hewitt using the contact information above.