CHICAGO—Aon Corp.’s Monday acquisition of Hewitt Associates Inc. will greatly boost Aon’s benefit consulting profile and make the company a major player in the benefits outsourcing market.

Chicago-based Aon said it would purchase Lincolnshire, Ill.-based Hewitt in a cash-and-stock-deal initially valued at $4.9 billion based on Friday’s close of Aon’s shares.

Aon said it expects to close the deal, which is subject to approval by federal antitrust regulators, in November.

The deal will bring Aon, the world’s second-largest insurance broker, increased strength as a benefit consultant.

The two firms reported about the same 2009 benefit consulting revenues. Hewitt had a little more than $1 billion in benefits consulting revenue in 2009, and Aon had just less than $1 billion. But Hewitt has a much greater penetration among the nation’s largest corporations than Aon, whose consulting strength is in the middle market.

“The combined client base will provide significant cross-sell opportunities to leverage Hewitt’s predominately large corporate client base with Aon’s predominately middle-market client base,” Aon said in a presentation to investors.

In benefits outsourcing there is no comparison. In 2009, Hewitt reported $1.5 billion in benefit outsourcing revenues, compared with $190 million for Aon. Benefit outsourcing comprises just over half of Hewitt’s consulting revenues, compared with 15% for Aon.

Both firms are known for their strengths in retirement and health care consulting. Hewitt is especially well-known for its expertise in communications consulting.

Following the close of the transaction, Aon said it intends to integrate Hewitt into its Aon Consulting unit and change its name to Aon Hewitt.

Russ Fradin, Hewitt’s chairman and CEO, will assume the same titles at Aon Hewitt, and report to Aon CEO Greg Case.

Aon said the transaction is expected to generate about $355 million in annual savings starting in 2013 through reducing overhead and leveraging technology platforms.

Bruce Ballentine, an analyst for Moody’s Investors Service in New York, said in a statement that the transaction will “sharply expand Aon’s capabilities” in consulting and resourcing, while also giving Aon a more balanced mix of brokerage and consulting revenues.

“The transaction would also improve the quality of earnings within Aon’s consulting segment, given that much of Hewitt’s revenue comes from multiyear outsourcing contracts,” Mr. Ballentine said.

On the other hand, negative aspects of the transaction include a doubling of Aon’s debt burden and “the potential for business disruptions during the integration phase,” Mr. Ballentine said.

Moody’s lowered its rating’s outlook on Aon debt to negative from stable.

The transaction comes almost exactly one year after Towers Perrin of Stamford, Conn., and Watson Wyatt Worldwide of Arlington, Va., announced what was then the largest merger of benefit consultants, with that deal valued at about $3.5 billion. That merger to establish Towers Watson & Co. was completed on Dec. 31, 2009.