Time resolved exhaust port sampling results show that the gas mixture in the port at exhaust valve closing contains high concentrations of hydrocarbons. These hydrocarbons are mixed with hot in-cylinder gases during blowdown and can react either via gas phase kinetics in the exhaust port/runner system or subsequently on the exhaust catalyst before they are emitted. Studies were conducted on a single cylinder, four stroke engine in our laboratory to determine the interaction between the hot blowdown gases and the hydrocarbons which remain in the exhaust port. A preselected concentration and volume of hydrocarbon tracers (propane, propene, n-butane, and 1-butene) in either oxygen/nitrogen mixtures or pure nitrogen were injected into the exhaust port just behind the exhaust valve to control the initial conditions for any potential oxidation in the port. The effects of port temperature, oxygen concentration, hydrocarbon concentration and volume, port mixing condition, fuel kinetics and equivalence ratio on port oxidation were studied. Our results directly verify the existence of hydrocarbon oxidation in the exhaust port. The port injection results with four tracer fuels show that the port oxidation scales with chemical reactivity. Studies with and without oxygen in the port injection show enhanced oxidation when local oxygen is present and indicate that limited mixing occurs in the exhaust port during blowdown. The studies identify physical, chemical and thermal limits on port oxidation. The studies also indicate a minimum port temperature of ∼1400 K for hydrocarbon oxidation as suggested in other experimental and modeling studies.