Exhibit 5.1


                          [Kirkland & Ellis Letterhead]




                                  April 4, 2001




Aon Corporation
123 North Wacker Drive
Chicago, Illinois 60606

                   Re: Shares of Common Stock, $1.00 par value

Ladies and Gentlemen:

      We are acting as counsel to Aon Corporation, a Delaware corporation (the
"Company"), in connection with the preparation and filing with the Securities
and Exchange Commission under the Securities Act of 1933, as amended (the
"Securities Act"), of a Registration Statement on Form S-4, Registration No.
333-57706 (the "Registration Statement") pertaining to the registration of a
proposed issuance by the Company of up to 3,852,184 shares of the Company's
Common Stock, $1.00 par value per share (the "Shares") to the stockholders of
ASI Solutions, Inc. ("ASI") in connection with the merger of a wholly-owned
subsidiary of the Company into ASI (the "Merger").

      We have examined originals, or copies certified or otherwise identified to
our satisfaction, of such documents, corporate records and other instruments as
we have deemed necessary for the purposes of this opinion, including the
following: (i) the Second Restated Certificate of Incorporation and the Bylaws
of the Company, each as amended to the date hereof; and (ii) certain resolutions
adopted by the Board of Directors of the Company. In addition, we have made such
other and further investigations as we have deemed necessary to enable us to
express the opinions hereinafter set forth.

      Based upon the foregoing and having regard to legal considerations that we
deem relevant, and subject to the comments and qualifications set forth below,
it is our opinion that the Shares being registered pursuant to the Registration
Statement, when issued to the stockholders of ASI in connection with the Merger,
will be duly authorized, fully paid and non-assessable.




      For purposes of this opinion, we have with your permission made the
following assumptions, in each case without independent verification: (i) the
authenticity of all documents submitted to us as originals, (ii) the conformity
to the originals of all documents submitted to us as copies; (iii) the
authenticity of the originals of all documents submitted to us as copies, (iv)
the genuineness of the signatures of persons signing all documents in connection
with which this opinion is rendered, (v) the authority of such persons signing
all documents on behalf of the parties thereto other than the Company and (vi)
the due authorization, execution and delivery of all documents by the parties
thereto other than the Company.

      We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to this firm under the section
entitled "Legal Matters" in the prospectus included in the Registration
Statement. In giving such consent, we do not thereby concede that we are within
the category of persons whose consent is required under Section 7 of the
Securities Act or the Rules and Regulations promulgated thereunder.

      We do not find it necessary for purposes of this opinion to cover, and
accordingly we do not purport to cover herein, the application of the securities
or "Blue Sky" laws of the various states to the offering and sale of the Shares.

      This opinion shall be limited to the general corporation law of the State
of Delaware.

                                                    Very truly yours,

                                                    /s/ Kirkland & Ellis

                                                    KIRKLAND & ELLIS