After sitting on the market for four months, the Blackstone Group has sold the Aon Center in Chicago for roughly $475 million, or $190 per sq. ft. Atlanta-based Wells Real Estate, an active buyer in 2002, has agreed to purchase the 2.5 million sq. ft. tower located at 200 East Randolph St. in Chicago’s East Loop district. It is the third largest building in the U.S.

With the purchase of the Aon Center, Wells has finally penetrated the Chicago market. In 2002, Wells bought the 420,000 sq. ft. Quaker Oats building at 555 West Monroe, then sold it to Prudential Real Estate Investors for nearly $100 million. Aon also is different than Wells’ typical office acquisitions of stable properties that are net-leased to single tenants. British-based oil company BP leases 600,00 sq. ft. within the Aon Center, and the company’s lease expires in a few years.

The 30-year-old Aon Center, formerly known as the Amoco Building, was purchased by the Blackstone Group in 1998 for $440 million. According to Real Estate Alert, institutional investors showed little interest in the Aon Center from the moment it hit the market. The building reportedly received only two formal bids last year.

Other recent acquisitions by Wells include the $157 million purchase of Nestle’s 545,000 sq. ft. headquarters in Glendale, Calif., and the purchase of a 949,000 sq. ft. Washington, D.C., office building with long-term government leases. Real Estate Alert reports that Wells typically invests through joint ventures where it holds a 90% stake.