A spokesman for its owner of the 83-story building, New York City-based Blackstone Real Estate Advisors, confirms the building is on the market, with Secured Capital Corp. brokering the sale. Officials of the New York City-based brokerage did not return telephone calls.

Meanwhile, a spokeswoman for Jones Lang LaSalle declined to confirm or deny reports her company is handling the sales effort for Prudential Plaza, located immediately west of Aon Center, for San Francisco-based Shorenstein Co., LP. The package includes 41-story, 1.76-million-sf One Prudential Plaza at 130 E. Randolph St. and 64-story, 1.4-million-sf Two Prudential Plaza at 180 N. Stetson Ave.

Blackstone paid $431 million for Aon Center in December 1998, financing the purchase with a $335-million mortgage. Shorenstein paid $372.6 million for Prudential Plaza in June 2000,with Fleet National Bank financing the sale with a $240-million mortgage. The sale raises eyebrows for reasons beyond the somewhat short holding periods.

"The biggest concern is they'll be competing against each other," one Downtown office broker tells GlobeSt.com. "It'll be difficult, because it will all be on the market at once." The broker, speaking on condition of anonymity, tells GlobeSt.com few buyers are players for the large assets that combine for nearly 6 million sf, and prices likely to total $1 billion.

Recent West Loop office sales prices are in the neighborhood of $250 per sf, which applied to the East Loop assets, would result in $675 million for Aon Center, $440 million for the original Prudential Building and $350 million for the newer Two Prudential Plaza.

However, the East Loop submarket has been among the harder hit by vacancies, with the buildings in play being chief among them. Deloitte & Touche LLP is leaving Aon Center, built in 1974, and Two Prudential Plaza in favor of a 424,000-sf lease in a building being developed by The John Buck Co. at 111 S. Wacker Dr.

Also, nearby Illinois Center properties have been trading closer to $175 per sf, which would drop the price of Aon Center to less than $500 million; $550 million for the Prudential Plaza buildings.