On-road gasoline and diesel-powered vehicle emissions in Nashville, Tennessee, were characterized using fuel sales as a measure of vehicle activity, and emission factors derived from infrared remote sensing, ambient air concentration ratios, and roadway tunnel measurements. On-road vehicle emissions of carbon monoxide (CO), nonmethane hydrocarbons (NMHC), and oxides of nitrogen (NOx) on weekdays during summer 1995 were estimated to be 270¿60, 43¿13, and 53¿9 metric tons per day, respectively. Diesel engines were a minor source of CO and NMHC, but were responsible for ~50% of NOx emissions from on-road vehicles. The Environmental Protection Agency's MOBILE 5B emission model predictions were similar to fuel-based estimates for all pollutants, except for NOx where the MOBILE model predicted a smaller contribution to total on-road vehicle emissions from diesel engines. Chemical composition profiles for hydrocarbon emissions were developed based on tunnel air and fuel samples collected in Nashville during summers 1995 and 1999. More than half of the tunnel NMHC mass was liquid fuel that escaped combustion; the remaining mass came from products of incomplete combustion such as ethane, acetylene, C2-C4 alkenes, and 1,3-butadiene, No major changes in the composition of vehicle-related NMHC emissions were observed between 1995 and 1999 in Nashville. ¿ 2001 American Geophysical Union