The pending sale is part of Ballmer's effort to diversify his personal holdings, said Microsoft, which noted that he owned about 471 million shares in the company as of September 2002. The exact number of the shares he is selling is expected to be released as early as Friday in a Securities and Exchange Commission filing.
While Microsoft co-founderstheir Microsoft holdings on a regular basis, Ballmer has taken a much more tentative approach.
"It's pretty amazing he hasn't sold any shares since May of 1991," said Lon Gerber, director of insider research at Thomson Financial. "Gates sells huge amounts all the time. But it's pretty shocking Ballmer hasn't sold any shares for over a decade."
Ballmer sold shares in May 1991, and throughout 1990, Gerber said. Before that, he sold shares in 1986, which was the first time he ever divested Microsoft holdings, Gerber said.
"Even though this is a personal financial matter, I want to be clear about this to avoid any confusion," Ballmer said, in a statement. "I remain excited about the potential for our technology to change people's lives, and I remain as committed to Microsoft as ever."
Rachel Wayne, a Microsoft spokeswoman, said the company will give no further information on Ballmer's decision and why he has waited until now to sell.
Shares of Microsoft were up 27 cents, or 1.12 percent, at $24.43 in morning trading.