Both the mechanism of individual hydrocarbon emission and the effects of engine variables on the individual hydrocarbon concentrations in the exhaust were investigated using a laboratory, single-cylinder engine with propane as the fuel. Individual hydrocarbon concentrations were measured, with a gas chromatograph, in samples obtained from the combustion chamber as well as from the exhaust system.Results indicate that, at the conditions investigated, a source of each of the individual hydrocarbons in the exhaust is the reaction (or lack of reaction) which occurs near the wall either at the instant the flame is quenched or immediately thereafter; however, cracking of the fuel hydrocarbon in the chamber after flame passage and in the exhaust pipe may also be a source of the non-fuel hydrocarbons. Relative proportions of fuel and non-fuel hydrocarbons in the exhaust are very sensitive to engine operating conditions.Total hydrocarbon concentration changes resulting from engine variable changes do not necessarily reflect, even directionally, changes in the individual hydrocarbon concentrations in the exhaust. Significant interactions exist among some of the engine variables: an engine variable effect observed at one air-fuel ratio, ignition-timing combination is not necessarily directionally the same as that observed at another air-fuel ratio, ignition-timing combination. Most of the engine variable effects can be qualitatively explained on the basis of the exhaust hydrocarbon emission mechanism discussed.