G.M., however, sees enough in Coskata's technology that it is buying an undisclosed amount of shares in the company's series B round of financing. Coskata's earlier financing has come from Khosla's Khosla Ventures, ATV and Greatpoint Energy. 

In addition to its financial investment, G.M. plans to use fuel produced by Coskata's pilot plant at its Milford Proving Grounds facility to test how well it performs in vehicles. The two firms say the pilot plant, likely to be built in the Midwest, will be operational by the end of 2008.

"This is a pretty big move for us," said Mary Beth Stanek, G.M.'s director of environmental and energy policy and commercialization. "It's a tighter relationship so that we can commercialize the idea more quickly."

She spoke along with Coskata's president and C.E.O. William Roe during an Internet web conference to brief journalists prior to Sunday's announcement. Journalists attending were required to agree to an embargo on the information for noon today -- heightening the sense that the announcement was important.

G.M.'s vision of itself as a leader in the environment has evolved dramatically during the past decade. In the mid-1990s, it continued to build larger, less fuel-efficient vehicles with higher profit margins even as its competitors' put them on their drawing boards.

Stanek calls recent moves a crystallization of the projects that G.M. has invested in over the years, not necessarily a shift in strategy. "We are still as hot on hydrogen as we have been," she said. "But now we are moving hybrids, plug-in electrics, and fuel cells out of research and development, and commercializing them."

Today, G.M. is dabbling in almost every green technology on the market. But a main stalling point for many of the ventures -- and one that may include Coskata -- is that turning great technological ideas into real cars or economically viable fuel alternatives has proven to be tough and time-consuming.

The company has been touting its Chevy volt all-electric vehicle for more than a year, but still hasn't worked out the bugs in the battery technology. The Autonomy fuel-cell project has been around for years and is just now showing evidence of moving beyond the lab. Even E85 has had problems in terms of creating the infrastructure to deliver the fuel to consumers with flex-fuel vehicles.

But G.M. can't stop making bets, given the pressures it faces from competitors, government, and even consumers.

Toyota, now a serious contender for G.M.'s long-standing rank as No. 1 car company in the world, assumed the mantle of environmentalism much earlier than G.M. in part because of the success of the Prius.