“We literally looked at every option we could think of, with several key parameters in mind,” said Emil Jacobs, vice president for research and development at Exxon’s research and engineering unit. “Scale was the first. For transportation fuels, if you can’t see whether you can scale a technology up, then you have to question whether you need to be involved at all.”

He added, “I am not going to sugarcoat this  this is not going to be easy.” Any large-scale commercial plants to produce algae-based fuels are at least 5 to 10 years away, Dr. Jacobs said.

Exxon’s sincerity and commitment will almost certainly be questioned by its most galvanized environmentalist critics, especially when compared with the company’s extraordinary profits from petroleum in recent years.

“Research is great, but we need to see new products in the market,” Kert Davies, the research director at Greenpeace, said. “We’ve always said that major oil companies have to be involved. But the question is whether companies are simply paying lip service to something or whether they are putting their weight and power behind it.”

But if it proves a bona fide effort, Exxon’s move into biofuels, long the preserve of venture capital firms and biotech start-ups, could provide a big push to the Obama administration’s policy of encouraging more renewable energy.