An analytical model of the hydrocarbon oxidation processes in an engine exhaust port was developed. A thermodynamic analysis was used to obtain the exhaust gas conditions as a function of lime and axial distance in the port. The major features of the exhaust port processes were described by submodels for the pan heat transfer, reaction rates. and instantaneous and overall levels of hydrocarbon concentrations as functions of engine conditions. Experimental determinations of the fraction of hydrocarbons oxidized in the exhaust port were obtained in engine experiments by the injection of a quench gas at selected locations within the port. By differencing the hydrocarbon concentrations observed during quenching and non-quenching operation, the degree of hydrocarbon oxidation was determined for each location. Analytical and experimental results were in good agreement for a range of engine operating conditions. The fraction of hydrocarbons reacted in the exhaust port ranged between about 2 and 70 percent for the various engine conditions examined for both the model and the experiment. These results were dominated by changes of gas temperatures, port residence times and oxygen concentration which occurred due to variations in the engine operating conditions and due to the injection of secondary air. The model was also used to examine hydrocarbon oxidation in the port as a function of cylinder gas temperature, port wall temperature, port length and port cross-sectional area.