Allen has filed to sell 9.65 million shares during the past month. Those shares represent roughly 5 percent of his 181 million shares, following the company's 2-for-1 stock split last month.
Given Microsoft's closing price of 82-5/16 today, the shares Allen is putting up for sale could bring in $794.3 million.
Gates, meanwhile, has filed to sell 1.3 million of his shares during the past month, which represents a fraction of his 540 million shares following the stock split. His pending sale could raise around $107 million, based on today's closing price.
Microsoft issued a 2-for-1 stock split in February, marking the seventh time the company has split its shares since it went public in 1986. The stock had been trading around 138 a share at the time of the announcement and has been climbing steadily since early January, when it traded in the low 60s.
"Allen is a regular seller," said Craig Columbus, an insider analyst with Disclosure. He noted that the Microsoft founder sold upwards of 5.8 million shares last August. (Allen is also an investor in CNET: The Computer Network.)
Susan Pierson, a spokeswoman for Allen, confirmed that he divests some portion of his Microsoft stake about every quarter.
"It's an overall diversification of his portfolio," she said. "It's been fairly consistent."
Bob Gabele, editor of Insiders Chronicle, said Gates's pending sale also is consistent.
"This is part of a regular program where he periodically sells a fraction of his holdings as part of a diversification plan," said Tom Pilla, a Microsoft spokesman. "This most recent activity is part of a very normal sale."
Other Microsoft insiders who have filed to sell stock include chief technology officer Nathan Myhrvold, who plans to unload 325,000 shares, and senior vice president Craig Mundie, who plans to sell 25,500 shares.