The loss of efficiency of oxidizing catalysts, which probably will be required in the future to meet vehicle emission control standards, is of prime concern. An investigation of the loss of catalytic activity resulting from exposure to potential catalyst poisons has been made utilizing engine-dynamometer tests and vehicle field tests. Tentative results indicate that Zinc, Barium, Calcium, and Magnesium additives in oil and the sulfur level in fuel do not have an apparent effect on catalyst deterioration. Low levels of lead and phosphorus in fuel do have an apparent effect. SE engine oils A, B, and C have not shown significantly greater deterioration effects than the ashless oil. The investigation of the effects of fuels and engine oils on catalyst deterioration is being continued.