A commercially prepared catalyst which uses Pd technology and which had been vehicle-aged for 56 000 miles was characterized for three-way catalytic performance in a laboratory reactor under conditions simulating the operation of the catalyst in a vehicle. The tests were conducted with the concentration of SO2 in the simulated exhaust feedstream set at 1 ppm, 5 ppm and 30 ppm (corresponding to 15, 75 and 450 ppm in fuel, respectively) to evaluate the magnitude and reversibility of the effect of sulfur on HC, CO and NOx activity. Tests were conducted using a propene/propane mixture as a surrogate for the hydrocarbon mixture in exhaust. The lightoff and warmed-up activities, particularly for HC and NOx, were found to be deteriorated when SO2 was added to the simulated exhaust feedstream, in agreement with previous work on the impact of sulfur on three-way catalysts conducted in this laboratory. In addition, the impact on lightoff and warmed-up activity of NOx and HC was found to be generally non-linear with sulfur concentration. The impact of sulfur on activity was found to be partially non-reversible at temperatures below 650 degrees C, but the original activity could be restored when the catalyst was operated at temperatures above 650 degrees C. Consistent with this result, the impact of sulfur on activity was found to be minimal for operation above 700 degrees C.