Two consumer-type vehicle tests to determine the effect of leaded and unleaded gasoline on exhaust emissions have been completed. One test involved 122 cars without exhaust control systems and the other, 36 cars with exhaust control systems. In both tests, hydrocarbon exhaust emissions of the leaded and unleaded cars increased during the initial period of mileage accumulation and then leveled out as equilibrium was reached. Average hydrocarbon emission levels of the leaded cars were higher than those of the unleaded cars with the difference or net lead effect amounting to 7% in both the 122-Carand the 36-Car tests. No significant differences in carbon monoxide or nitrogen oxide emission levels were observed. Photochemical reactivity levels were essentially the same for the leaded and unleaded car groups in the two tests.A limited study of the effect of mileage accumulation conditions on exhaust emission levels was carried out. Results obtained under rapid or accelerated mileage accumulation conditions did not correlate with consumer test results.The role of combustion chamber deposits in increasing hydrocarbon exhaust emission levels during the initial period of operation of a vehicle has been considered. Bulk volume of deposits appeared to be a better parameter than deposit weight for relating hydrocarbon exhaust emissions and deposit properties.