Absorption and desorption of fuel by motor oil that coats the cylinder side wall with a thin oil film is thought to be one of the important sources of unburnt hydrocarbon emissions from spark ignition engines. The experiments presented in this paper determine the effect of oil films on unburnt hydrocarbon emissions. The exhaust gas from a combustion bomb in the presence and absence of oil has been analyzed by gas chromatography. Experiments show that the exhaust hydrocarbon concentration increases when motor oil is applied on a surface of the bomb. This result shows that absorption of the fuel occurs in the oil prior to ignition, and dissolved fuel is then desorbed into the burnt gas after the completion of combustion.