Modern four-valve engines are running at ever higher compression ratios in order to improve fuel efficiency. Hotter cylinder bores also can produce increased fuel economy by decreasing friction due to less viscous oil layers. In this study changes in compression ratio and coolant temperature were investigated to quantify their effect on exhaust emissions. Tests were run on a single cylinder research engine with a port-deactivated 4-valve combustion chamber. Two compression ratios (9.15:1 and 10.0:1) were studied at three air/fuel ratios (12.5, 14.6 and 16.5) at a part load condition (1500 rpm, 3.8 bar IMEP). The effect of coolant temperature (66 °C and 108°C) was studied at the higher compression ratio. The exhaust was sampled and analyzed for both total and speciated hydrocarbons. The speciation analysis provided concentration data for hydrocarbons present in the exhaust containing twelve or fewer carbon atoms. The specific reactivity (g Ozone/g NMHC) of the exhaust was calculated for each operating condition. Total HC emissions increased by about 20% as the compression ratio was raised. The speciation results show that the percentage of fuel species was higher and the percentage of partial combustion products was lower at the higher compression ratio. This was due to more charge packing into the crevices and less oxidation with the higher compression ratio. The specific reactivity of the emissions was not significantly different for the two different compression ratios. The 42 °C coolant temperature increase produced about a 30% decrease in the total HC emissions. The higher temperature environment in the combustion chamber produced a higher percentage of partial combustion products with a decreased percentage of fuel hydrocarbons (especially higher molecular weight species). The specific reactivity was not changed with the change in coolant temperature. Modelling of the HC emission mechanisms was also performed. The model correctly predicted the effect of coolant temperature on the total HC emissions when the oil layer solubility was changed to give an oil layer contribution of 5% at the standard coolant temperature (91°C). The model predicted an 18% increase in the HC emissions as the compression ratio was changed from 9.15:1 to 10.0:1. This was due equally to increased charge packing in the crevice and decreased port oxidation.