The hydrogen-fueled hybrid electric/fuel-cell bus is one of only four in use in the United States, but promoters of fuel-cell technology hope that studies of the buses' performance will eventually lead to widespread use. The buses not only save gas. Water vapor, not smoke, comes out of their pipes, so they also reduce air pollution.

The transit agency began using the bus in December. This summer, SunLine is giving free rides on the bus to educate the public about its benefits. Explanatory leaflets and signs are inside. The bus runs on different routes every day, so riders never know when they'll be aboard.

Monica Mendoza, 30, was enjoying the ride down Highway 111 on a recent morning.

"It's smoother and softer, and it saves the environment," the Indio woman said.

At $3.1 million, the buses are still far too expensive for transit agencies to buy in bulk. The cost will drop as they become more common, said Thomas Blazak, an engineer with Poway-based ISE Corp., which designed the hybrid drive system. Standard diesel buses typically cost about $360,000, said C. Mikel Ogelsby, general manager of SunLine.

The transit agency didn't spend any money on the hydrogen bus. The federal government -- which wants to see how hydrogen buses perform -- paid the bulk of the cost. The three companies that built the bus and the South Coast Air Quality Management District also contributed.

For years, SunLine has promoted alternative fuels. It switched its entire fleet to natural-gas buses in 1994 and it started using a near-zero-emission bus in 2004. That bus, which has a hydrogen-fueled internal combustion engine, is less efficient than the new model, so SunLine must spend more money on natural gas to create the additional hydrogen, said Tommy Edwards, director of maintenance for SunLine.

SunLine creates compressed hydrogen at its Thousand Palms headquarters with solar energy or natural gas. The compressed hydrogen is then pumped into the bus' fuel cells, in which the hydrogen mixes with filtered oxygen sucked in from the atmosphere. The fuel cells -- which are like a giant battery -- then create electricity through a chemical reaction. Water exits the bus in the form of steam.

The three other hybrid electric-fuel-cell buses in the United States are on AC Transit routes in the Oakland-Berkeley area. The Santa Clara Valley Transportation Authority runs five zero-emission hydrogen buses, but because they are not hybrids, they aren't as efficient as the one SunLine operates, said Jaimie Levin, director of alternative fuel policy for AC Transit. Other hydrogen-fueled buses run in Europe, Asia and Australia.

In addition to the nine hydrogen-powered buses on the road in California, there are 134 hydrogen-powered passenger cars, said Chris White, spokeswoman for the California Fuel Cell Partnership, a government-industry consortium. The state, several cities and companies such as UPS are among the operators of those cars, she said.

The state's 23rd hydrogen-fueling station for cars opened June 15 in Santa Monica. Inland hydrogen-fueling stations are in Riverside, Chino, Ontario and at SunLine's Thousand Palms site. Fifteen more stations are expected to open in the next year, White said.