Optimized 1997 model year Caterpillar C10 dual-fuel natural gas engines certified to the California Air Resources Board's Alternative Low NOx 2.5 gram/brake horsepower-hour emission standard were demonstrated in three commuter buses over a 12-month period, in Santa Barbara, California. The project evaluated the retrofit costs and process, performance, reliability, fuel economy, operating costs, and emissions of the three C-10 dual-fuel natural gas engines compared to a standard C-10 diesel engine. Chassis dynamometer tests using the U.S. EPA Urban Dynamometer Drive Schedule, the Central Business District (West Virginia University version) and the 55-mph Steady State cycles were conducted to characterize in-use emissions of the dual-fuel engines for the commuter bus application.During 94,000 combined service miles, performance, reliability and durability of the dual fuel buses were similar to the diesel control. Compared to the diesel control, the C-10 dual fuel natural gas engine operating in the dual fuel mode had 27 to 60% average lower NOx, PM and CO2 (14 to 19%) emissions for all cycles. CO and NMHC emissions were higher than expected. CNG use was two-thirds the expected level and could be improved. Fuel economy was within 11 percent of the diesel fuel economy. Operating costs for the dual-fuel buses were nearly twice the diesel bus costs due to the CNG component costs, lower fuel economy and refueling costs. This cost difference can be eliminated over a longer operating period if CNG prices are less than diesel fuel prices. This report documents how the Caterpillar C10 dual fuel natural gas engines performed in a commuter bus application during 94,000 miles of service.