Their consensus: Hydrogen, a gas derived from water, holds perhaps the greatest potential for replacing oil as the lifeblood of the world economy -- but not for years, possibly decades.

"It isn't pie-in-the-sky,'' said Catherine Dunwoody, executive director of the California Fuel Cell Partnership, at the event staged by the California Air Resources Board. "It certainly is in the future, but it's something that we have to be working on today.''

Already hydrogen is a $50 billion-a-year global commodity used for manufacturing oil products, steel, glass and other things. Making it available to the average driver, however, will require a huge investment in infrastructure related to the storage and distribution of hydrogen to the mass market.

Representatives from BMW of North America, General Motors Corp. and Honda Motor Co. all touted their efforts to develop hydrogen vehicles for a mass market. A limited number of hydrogen vehicles already exist, but they are typically government fleet vehicles, municipal buses or experimental vehicles.

"We at GM believe in the hydrogen economy, and we think it's coming sooner rather than later,'' said Al Weverstad, General Motors' mobile emissions and fuel efficiency director and chairman of the California Fuel Cell Partnership,

But Weverstad did not predict his firm would be prepared to even consider large-scale production until at least 2010.

Wilhelm Hall of BMW of North America said his company plans within three years to deliver a version of its 7-series vehicles that will be capable of using either gasoline or hydrogen as fuel.

In addition to improving the performance of existing hydrogen-powered cars, the question of how to efficiently store and distribute hydrogen -- a bulky gas usually stored under pressure -- presents enormous challenges, although there is disagreement on how far technology has to go to be viable.

"That's an area that we find problematic,'' said Jeffrey Jacobs, vice president of the hydrogen unit at Chevron Technology Ventures LLC in San Ramon. "The transition period is going to be slow.''

Tim Lipman, an assistant research scientist at UC Berkeley's California Center for Innovative Technologies, said automobile technology may be ready by 2010, but the refueling infrastructure will not.

Lipman was nevertheless optimistic about the prospects for hydrogen, particularly given the sustained commitment of large corporate players like Chevron and automakers.

"A lot of money is going into this,'' Lipman said. "That's why a lot of us think this is going to happen, because it's not something that is being driven by governments. There are large investments by oil and automobile companies.''

Hydrogen fuel has allies elsewhere as well. Gov. Arnold Schwarzenegger is among its biggest champions and has called for a "hydrogen highway" of fuel cell filling stations to be established across the state.

Some critics of fuel cell technology, however, say that because the production of hydrogen is a byproduct of the gasoline refining process, the green benefits of nonpolluting fuel cell cars are canceled out. But from an environmental perspective, Dunwoody said, virtually any scenario involving hydrogen would be superior to continued dependence on oil.

No matter how hydrogen is produced, whether with natural gas, electricity off the California grid or solar power, emissions of greenhouse gases are significantly lowered when compared with continued use of gasoline, even with hybrid electric-gasoline cars, various experts said.

Steve Ellis of the Honda Corp. said that using fossil fuel to manufacture hydrogen will be necessary during the transition to a hydrogen economy.

"Expecting green hydrogen out of the chute would be like expecting a newborn infant to walk out of the hospital and change its own diaper,'' he said.

In the long run, said Jon Slangerup, chief executive officer of Solar Integrated Technologies, renewable electricity will be the preferred method for manufacturing hydrogen.

"The technologies to deliver this are well understood and mature," he said. "We're just waiting for the vehicles to come to market.''