A study was made of the deactivation of a pelletized base metal oxide catalyst which was evaluated in an eight-car fleet for accumulation of up to 18,000 miles in customer-type driving. The fleet test was carried out to determine the effects of two converter locations and four tetraethyl lead levels (0.06-3.17 g/gal) in the fuel on the effectiveness of the catalyst for the control of CO and HC emissions. Samples of the used catalysts were examined to ascertain the changes in BET surface area, size distribution and chemical composition of the contaminants deposited on the catalyst, and the patterns of penetration of lead contaminant. The major changes found in the used catalyst samples were correlated with the activity data obtained by the catalyst manufacturer on duplicate catalyst samples.The amount of lead deposits found on the catalyst represented about 50% of the lead in the fuel based on an assumed gas mileage of 17 mpg, and was nearly in direct proportion to the lead content in the fuel and independent of converter location. The catalyst activity for CO and HC oxidation was adversely affected by the accumulation of lead deposits.The major effect due to the converter locations investigated in this study was found in the loss in the catalyst BET surface area. Catalyst samples from front-mounted converters suffered a greater loss of surface area, but a nearly twofold variation in catalyst surface area (50 versus 100 m2/g) had a relatively small effect on the HC oxidation activity.