Exhaust gases from fourteen 1970-4 model and twenty 1975-7 model General Motors cars were collected during 1975-8 Federal Test Procedure tests and analyzed by gas chromatography. Hydrocarbon reactivity was calculated from the chromatographic analyses, using several reactivity scales.The use of oxidation catalytic converters on the 1975-7 model cars greatly changed the exhaust hydrocarbon composition in comparison to 1970-4 model cars. In general, such use caused individual paraffins to increase in carbon percent and individual olefins and acetylene to decrease. For example, the methane carbon percent was 5.0 for 1970-4 model cars (nonconverter cars) and 14.5 for converter cars; ethylene percent was 12.5 for nonconverter cars and 7.4 for converter cars; propylene was 6.5 for nonconverter cars and 2.9 for converter cars; and acetylene was 7.9 for nonconverter cars and 2.2 for converter cars. Because of these large changes in hydrocarbon composition, each of the reactivity scales evaluated indicated that converter cars produced exhaust hydrocarbon mixtures that were less reactive than those of nonconverter cars. The reductions in reactivity per gram ranged from about 10 to 35 percent.