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Factors influencing the effectiveness of exhaust port air injection in oxidizing the hydrocarbons and carbon monoxide in engine exhaust gas have been investigated in order to establish guidelines for the engineering of vehicle emission control systems. Single-cylinder engine and vehicle studies have demonstrated that the temperature, composition, and residence time of the exhaust gas-air mixture are basic factors determining both the effectiveness of air injection and the type of oxidation process which occurs in the exhaust system. Both luminous and nonluminous oxidation have been observed. These basic factors are affected by such variables as: engine spark timing and air-fuel ratio, insulation and size of exhaust manifolds, injection air temperature and airflow rate, and the warmup characteristics of the air injection system. The warmup characteristics can be influenced particularly by spark timing and exhaust manifold design.