CHICAGO—Aon Corp. on Friday completed its acquisition of Hewitt Associates Inc., a record deal that will significantly boost Aon’s resources as a benefit consulting and outsourcing provider.

In the biggest deal ever involving a benefit consultant, Aon acquired Hewitt in a cash-and-stock deal valued at $4.9 billion based on the July 9 close of Aon’s shares.

The deal eclipses last year's merger of Towers, Perrin, Forster & Crosby Inc. and Watson Wyatt Worldwide Inc., a $3.5 billion transaction that was the previous record merger of benefit consultants.

“The completion of this merger marks yet another important milestone in the history of Aon and is an industry-changing event that will create new standards in the human capital space,” Aon Corp. President and CEO Greg Case said in statement.

Chicago-based Aon will integrate Hewitt and its Aon Consulting unit and has changed its name to Aon Hewitt Inc. Aon Hewitt's chairman and CEO is Russ Fradin, who held the same titles at Lincolnshire, Ill.-based Hewitt.

A decision has not yet been made on where Aon Hewitt will be based, an Aon spokesman said.

While Hewitt and Aon Consulting each generated just over $1 billion in consulting revenues last year, their market bases were strikingly different. Hewitt was a dominant player in the large-employer market, with more than half of Fortune 500 companies among its clients.

By contrast, Aon Consulting's business was concentrated among middle-market employers—organizations with fewer than 5,000 employees.

“While Aon is very strong in the middle market, acquiring Hewitt will significantly expand our ability to serve large employers,” Aon Consulting CEO Kathryn Hayley said at the time Aon announced the deal in July. Ms. Hayley now is CEO of consulting-Americas for Aon Hewitt.

In the outsourcing field, Hewitt's $2 billion in revenues were about 10 times that of Aon Consulting.

In addition, a combined Aon Hewitt will bring more balance to Aon's book of business, Ms. Hayley said.

Based on fiscal 2009 figures, 49% of Aon Hewitt's revenues would be from consulting services, 40% from benefit outsourcing and 11% from human resources business process outsourcing. By contrast, about 85% of Aon Consulting revenues were derived from consulting in fiscal 2009.

Prior to the transaction, Hewitt, which was founded in 1940, had about 23,000 employees, while Aon Consulting had 6,300 employees and parent Aon Corp. had about 36,000.