As part of a study to ascertain the feasibility of hybrid heat engine/electric automotive vehicles, exhaust emissions were calculated to determine both the state-of-the-art and the potential emission capability of vehicles incorporating the battery/heat engine powerplant. Vehicle emissions were determined for four vehicle classes: full-size family car, small commuter car, delivery/postal van, and the city bus. Specific mass emission data (in terms of gm/bhp-hr) were gathered for the spark ignition, diesel, gas turbine, Rankine cycle, and the Stirling cycle engines. Data inputs required the determination of steady-state mass emission data at rated-load as well as part-load operating conditions. These data were utilized to calculate the exhaust emission characteristics of the passenger car and commuter car driving over the Federal LA-4 driving cycle. Emission data for the delivery van and city bus were calculated over an arbitrarily selected driving cycle. The results indicate that marked reductions in emissions can be obtained with the spark ignition engine operating at lean air-fuel ratios, which is permitted by the steady-state operation characteristic of the hybrid. For the future, the gas turbine shows promise of further emission reductions, and the diesel engine shows promise in certain areas.