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This study describes the changes in the chemical composition of the exhaust at various equivalence ratios (air-fuel weight ratios) when pure isooctane, diisobutylene, toluene, and toluene-n-heptane mixture are used as fuels in a single-cylinder engine, operating at constant conditions. Isooctane and diisobutylene combustion produced large concentrations of olefins in the exhaust, while toluene produced small amounts of ethylene as its only olefinic product. The olefins, in general, showed a strong temperature dependence and exhibited maximum emissions near the stoichiometric equivalence ratio. Combustion of the mixture of 25 volume percent n-heptane in toluene reveals interesting information, compared to emissions from pure toluene: concentrations of ethyl-benzene, styrene, and dimethylacetylene surprisingly are increased by factors of 1.9, 1.9, and 2.1, respectively, probably because reactive radicals derived from h-heptane interact with toluene to form unsaturated molecules.