Catalysts taken from the lightoff and underfloor converters of two 1986 Corvette exhaust systems which had been vehicle-aged for 100,000+ miles were cut into 1″ thick sections along their axis and characterized for lean lightoff and warmed-up performance using a laboratory reactor. Sections were then treated to remove the poisons, and the characterization was repeated. An axial gradient in both lightoff and warmed-up conversion efficiency for HC and CO was detected within the first 2″ of both lightoff converters for both vehicles. This activity gradient is in agreement with the gradient in the phosphorus and zinc concentrations found in the front 2-3″ sections of the lightoff converters. Comparison of these data sets with those obtained after removal of the P and Zn poisons led us to conclude that a significant part of the total deterioration of activity can be attributed to phosphorus and zinc poisoning for the front 1″ (and to a lesser extent for the second 1″ section) of the lightoff converters.