The chemistry of unburned hydrocarbon oxidation in SI engine exhaust was modeled as a function of temperature and concentration of unburned gas for lean and rich mixtures. Detailed chemical kinetic mechanisms were used to model isothermal reactions of unburned fuel/air mixture in an environment of burned gases at atmospheric pressure. Simulations were performed using five pure fuels (methane, ethane, propane, n-butane and toluene) for which chemical kinetic mechanisms and steady state hydrocarbon (HC) emissions data were available. A correlation is seen between reaction rates and HC emissions for different fuels. Calculated relative amounts of intermediate oxidation products are shown to be consistent with experimental measurements.