Gaseous emissions were sampled from the exhaust of a single cylinder version of a modern four-valve homogeneous charge spark-ignition engine. The hydrocarbon emissions were extensively analysed using capillary gas chromatography. Levels of key components of the hydrocarbons including methane, benzene and 1, 3-butadiene, were related to fuel composition, mixture strength and exhaust gas recirculation rate. It was shown that the relative levels of hydrocarbon emissions could generally be explained from a knowledge of chemical mechanisms. The significance of the observed trends for the development of engines with reduced levels of hydrocarbon emissions is considered.