A program has been completed to study the effects of ambient temperature on exhaust emissions. It consisted of determining the cold start emission levels of two late model passenger cars at temperatures of +70, +40, and -10°F with two test fuels of different midrange volatility characteristics.The ambient temperature was found to have a pronounced effect on exhaust emissions. A reduction of the test temperature from +70 to -10°F resulted in a manyfold increase in hydrocarbon and carbon monoxide emissions. Most of this increase in emissions, as well as an associated deterioration in fuel economy, occurred during the initial 5 min period after a cold start. The ambient temperature was found to have no major effect on the levels of pollutants emitted during the fully warmed-up operation of cars.Relative to the effects of ambient temperature and the vehicle characteristics, the midrange volatility of the fuel was concluded to have only a minor effect on exhaust emissions.