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This paper discusses the effect of poisons originated from the engine oil and fuel on the catalytic activity of palladium based close coupled catalysts. The performances of seven aged catalysts were analyzed by synthesized gas activity tests. The effects of thermal deactivation of the catalyst were investigated by BET surface area and X-ray diffraction tests. To show the poisoning effects along the exhaust stream, the additives of oil and fuel were quantified by ICP spectrometry and x-ray flouresences (XRF). SEM-EPMA analysis were also performed to determine the distribution of poisons in the washcoat. The contribution of poisoning to the overall deactivation process could than be estimated by comparing the results before and after leaching the poisonous elements with a weak acid solution. Test results show the activity of the palladium based catalysts can be severely deteriorated by poisons.