The fate of hydrocarbon species in the exhaust systems of spark-ignition engines is an important part of the overall hydrocarbon emissions problem. In this investigation models were developed for the instantaneous heat transfer, fluid mixing, and hydrocarbon oxidation in an engine exhaust port. Experimental measurements were obtained for the instantaneous cylinder pressure and instantaneous gas temperature at the exhaust port exit for a range of engine operating conditions. These measurements were used to validate the heat transfer model and to provide data on the instantaneous cylinder gas state for a series of illustrative exhaust port hydrocarbon oxidation computations as a function of engine operating and design variables.During much of the exhaust process, the exhaust port heat transfer was dominated by large-scale fluid motion generated by the jet-like flow at the exhaust valve. A correlation based on the jet velocity through the valve opening was developed which correctly estimated the heat transfer due to this large-scale motion. A four-period heat transfer correlation for the complete exhaust process was then formulated, based on two distinct flow regimes, which provided good agreement with the measurements.For individual variations of the engine operating conditions, computed reductions in hydrocarbon levels due to oxidation in the exhaust port ranged between 9 and 38%. The effect of the operating conditions was governed primarily by the resultant variations in exhaust gas temperatures and port residence times. Variations in specific design parameters, such as exhaust port geometry and internal port insulation, changed the overall degree of hydrocarbon oxidation in the exhaust port.