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                                                                    Exhibit 99.1

                CERTIFICATION PURSUANT TO SECTION 1350 OF CHAPTER 63
                       OF TITLE 18 OF THE UNITED STATES CODE

     I, Patrick G. Ryan, the Chief Executive Officer of Aon Corporation (the
"COMPANY"), certify that (i) the Quarterly Report on Form 10-Q of the Company
for the quarterly period ended September 30, 2002 (the "REPORT") fully complies
with the requirements of Section 13(a) or 15(d), as applicable, of the
Securities Exchange Act of 1934, as amended; and (ii) the information contained
in the Report fairly presents, in all material respects, the financial condition
and results of operations of the Company.

                                                /s/ Patrick G. Ryan
                                                -----------------------------
                                                Patrick G. Ryan
                                                Chief Executive Officer
                                                November 14, 2002