The roughly $30.7 billion donation doubles the Gates Foundation's size to $60 billion, five times larger than any other U.S. charitable group and bigger than the gross domestic product of Kuwait.

Bill Gates, the world's richest person, co-founded and remains chairman of software company Microsoft Corp..

His friend and bridge partner Buffett, 75, became known as the Sage of Omaha as he built the world's second biggest personal fortune running Berkshire Hathaway Inc., an insurance and investment company.

"I am not an enthusiast for dynastic wealth, particularly when the alternative is 6 billion people having much poorer hands in life than we have," Buffett said at a signing ceremony with the Gateses at the New York Public Library.

Gates, 50, this month said he plans in July 2008 to step down from his day-to-day Microsoft role to focus on his foundation, one of the goals of which is to improve access to technology in U.S. public libraries. It also focuses on fighting diseases such as HIV/AIDS, tuberculosis and malaria.

Buffett pledged 10 million Berkshire Class B shares to the Gates Foundation. He also pledged 2.05 million Class B shares to foundations in the name of his late wife, Susan, who died in 2004, and for his three children, Susie, Howard and Peter.

The total donations of about $37.1 billion amount to 85 percent of Buffett's net worth and constitute the largest single act of U.S. charitable giving ever.

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