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The poisoning of diesel oxidation catalysts (DOCs) by the engine oil additive zinc dialkyldithiophosphate (ZDDP) is investigated in the present study. A 517cc single-cylinder diesel engine is used to accelerate the phosphorus poisoning of DOCs by artificially increasing the ZDDP consumption to approximately 700 times normal operation by three different methods. These include lube-oil doped fuel, intake manifold, and exhaust manifold injection with lube-oil containing an elevated level of ZDDP. The deactivation of DOCs under these conditions is characterized by a variety of physical and chemical techniques. Surface composition and structure of the poisoned catalysts analyzed with SEM-EDS show differences depending on the method of ZDDP introduction. Exhaust manifold injection produces a zinc phosphate glaze which masks the surface to species diffusion. Fuel and intake manifold injection methods produce chemically absorbed phosphorus on the catalyst washcoat surface.