The paper discusses a validation program aimed at confirming or modifying conversion factors used in EPA's MOBILE model, using actual on-road-measured emissions data. For the benefit of the emissions inventory community and regional air quality modelers, EPA's Office of Mobile Sources has produced a model, the latest version of which is called MOBILE5b, that predicts mobile source emission factors for ozone precursors and carbon monoxide (particulate exhaust emission factors are estimated with a similar, but separate, model, PART5). This model was developed using information on motor vehicle fleet characteristics, usage patterns, and the regulatory and economic forces that effect them. The final outputs from the MOBILE model are composite emission factors, usually expressed as grams/vehicle-mile of travel, in which all of these factors have been weighted together to produce average emission factors for each pollutant and each vehicle class and fuel type for a specific calendar year.