Microsoft CEO Steve Ballmer talks to PDC 2010 attendees about HTML5.

Microsoft CEO Steve Ballmer plans to sell up to 75 million Microsoft shares to help "gain financial diversification" and to "assist in tax planning before the end of the year," the company announced today.

Before taking into account recent sales, Ballmer held 408 million shares, the company said, citing the latest Microsoft proxy report.

"Even though this is a personal financial matter, I want to be clear about this to avoid any confusion," Ballmer said. "I am excited about our new products and the potential for our technology to change people's lives, and I remain fully committed to Microsoft and its success."

This is the first time in seven and a half years that Ballmer has sold stock. His previous sales of Microsoft stock back in 2003 came in just under $1.6 billion.

According to SEC filings, Ballmer has already sold off 49,341,652 shares of common stock across three different transactions this week totaling a little more than $1.3 billion. Coincidentally, Microsoft chairman and founder Bill Gates also sold some of his company stock this week with two different sales (1, 2) totaling an even 3 million shares.

The tax ramifications on the two sales are of special note given the savings and investment tax hikes that are going into effect in January. The maximum long-term capital gains tax rate will increase to 20 up from 15 percent. In Ballmer's case, that ends up being a savings of around $67 million by selling now instead of January, assuming Microsoft's stock price stays the same.

CNET's Ina Fried and Declan McCullagh contributed to this report.

Updated at 11:34 p.m. PDT with additional tax information and a different link to the tax increase reference.