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A reaction scheme depicting engine oil oxidation chemistry occurring in internal combustion engines is proposed. This scheme reflects the idea that hydroperoxides, which are initial oxidation products, are formed continuously in engine oil, regardless of the presence of radical trapping inhibitors, due to a continuous influx of free radicals from the combustion process. Therefore, the antioxidant behavior of a zinc dialkyldithiophosphate (ZDTP) itself and in combination with an ashless phenolic antioxidant has been investigated using a model hydrocarbon oxidation system in the presence of excess hydroperoxides. In order to approximate temperatures existing in critical engine locations, these studies were carried out at 160°C. Results obtained contribute to a better understanding of the antioxidant mechanisms of ZDTP and also provide basic information needed in development of laboratory test procedures for evaluation of engine oil antioxidant systems under realistic conditions.