Exhibit 10.1


Dear Dennis


This Letter Agreement
constitutes a variation of the Agreement between you and Aon Group INC. whose
registered office is situated at 200 East Randolph Street, Chicago, Illinois
60606, USA (the Group) and Aon Limited (formerly Aon Group Limited), 8
Devonshire Square, London, EC2M 4PL (the “Company”) dated 30 November 1998 (the
“Agreement”).


IT IS HEREBY AGREED THAT IN
CONSIDERATION OF THE COMPANY EXTENDING THE PERIOD OF YOUR AGREEMENT, THE
AGREEMENT WILL BE VARIED AS SET OUT BELOW. ALL OTHER TERMS AND CONDITIONS OF
THE AGREEMENT WILL CONTINUE TO APPLY:


1.                                       Clause
3 (A) of the Agreement shall be replaced with the following provision:


“This Agreement shall commence on 30 November 1998 and
shall supersede all or any existing agreements which may exist between the
Executive and any Group Company, and subject to the provisions for early
termination contained herein (including but not limited to Clause 14 of the
Agreement), the Executive’s employment shall continue until the Planned
Expiration Date as defined in paragraph 4 of this Letter Agreement unless:


(i) on or before 20 September 2012, the Agreement
shall be terminated by the Company giving to the Executive 36 months notice; or


(ii) after 20 September 2012, the Agreement shall be
terminated by the Company giving the Executive notice equal to the number of
days between the date of giving notice and the Planned Expiration Date; or


(iii) on or before 20 September 2010 (the “Earlier
Termination Date”), the Agreement shall be terminated by the Executive giving
to the Company 12 months’ written notice; or


(iv) after 20 September 2010 up until the Planned
Expiration Date, the Agreement shall be terminated by the Executive giving to
the Company 6 months written notice expiring at any time before the Planned
Expiration Date.”


2                                        Clause
3 (B) of the Agreement shall continue to apply only up until the Earlier
Termination Date. Between the 21 September 2010 and the Planned Expiration
Date, Clauses 3 (B) (i), (ii), (iii), (iv) and (v), and (C) of the Agreement
shall be replaced with the following provision:


“Notwithstanding anything
to the contrary in sub-clause 3 (A) above, the Company may make a payment of
basic salary only in lieu of notice for a period equivalent to the applicable
notice period referred to in Clause 3 (A). The Company will not make any
provision for the continuation of any additional benefits as set out in
paragraphs 15, 16 and 17 of the Schedule to the Agreement (or for payment in
lieu thereof) or for the provision of any car allowance, pension or bonus for
the period equivalent to the notice period referred to in Clause 3 (A) (or for
payment in lieu thereof).”


3.     Clause 3 (D) of the Agreement shall be
replaced with the following provision:









 



Subject to Paragraph 2 of this Letter Agreement the
Executive shall be entitled to participate in any bonus scheme as described in
4 (C) below, on a pro-rata basis in his final year of service under this
Agreement, should his final date of employment or date of notice under
Paragraph 1 of this Letter Agreement, whichever is earlier, in his final year
of service under this Agreement fall other than the date on which the bonus is
calculated.


4.                                       In
variation of Paragraphs 3 and 5 of the Schedule to the Agreement, the Planned
Expiration Date shall be 20 September 2015 and the Normal Retirement Date 20
September 2010.


5.                                     In
variation of Paragraph 15 of the Schedule to the Agreement, the following shall
be added as Paragraph 15 (f) of this Schedule:


With effect from the date of this Letter Agreement the
Company will pay interest on a mortgage on the Executive’s principal residence
of up to £120,000 per annum directly to the lender.  Interest is payable up to the Executive’s
60th birthday or termination of your contract of employment if earlier.


End — next page is
signature page









 



IN
WITNESS whereof this variation to the Agreement has been
entered into by:


SIGNED
for and on behalf of Aon Group, Inc. by:


	
  President and Chief
  Executive Officer

  	
  /s/ Gregory C. Case

  	
   

  



 


GREGORY
C CASE


SIGNED
for and on behalf of Aon Limited by:


	
  Director

  	
  /s/ David P. Bolger

  	
   

  



 


Signed as a deed
and delivered by the said Executive


	
  /s/ Dennis L. Mahoney

  	
   

  



 


MR
DENNIS LEONARD MAHONEY


	
  Date 

  	
  July 19, 2006

  	
   

  



 


	
   

  	
  In the presence of:

  
	
   

  	
   

  	
   

  	
   

  
	
   

  	
  Name

  	
  Jeremy Farmer

  	
   

  
	
   

  	
   

  	
   

  	
   

  
	
   

  	
  Address:

  	
  Aon Corporation

  	
   

  
	
   

  	
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  200 E. Randolph St.

  	
   

  
	
   

  	
   

  	
   

  	
   

  
	
   

  	
   

  	
  Chicago, IL 60601

  	
   

  
	
   

  	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  	
   

  
	
   

  	
  Occupation

  	
  Senior Vice President — Aon Corporation