A single-cylinder, two-valve engine was operated with independent cooling circuits for the engine block and cylinder head to investigate the effect of temperature distribution on HC emissions. The goal was to understand and quantify the mechanisms responsible for decreased HC emissions at elevated temperatures. Tests were run at a typical road load condition using two different fuels (a 97 RON blend and isopentane - to eliminate liquid fuel and oil layer absorption effects). The total HC emissions (97 RON fuel) decreased by 15-20% with an increase in either the cylinder head or engine block coolant temperature from 71 to 110 °C. When operating with isopentane the HC emissions decreased by 15-20% with an increase in the engine block temperature from 71 to 110 °C but were essentially unaffected by cylinder head temperature. This indicates that the cylinder head temperature primarily influenced the HC emissions from liquid fuel effects. The exhaust gas temperature did not change significantly with an increase in either the cylinder head or engine block temperature so it is believed that port oxidation was not significantly affected. The increased cylinder block coolant temperature produced decreased crevice loading (lower gas density) as the liner and piston temperatures increased. An increase in the liner temperature should also reduce the density of the wall layer gas which expands from the piston crevice and increase the diffusion into the hot burned gases, increasing in-cylinder oxidation. The results indicate that the effect of oil layer absorption on the HC emissions was not large since the liner temperature had a similar effect on the emissions when operating with either a soluble (97 RON) or a relatively insoluble (isopentane) fuel.