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This is a continuation of our earlier paper on the laboratory evaluation of three-way catalysts, SAE 76201. A number of recent 3-way catalyst formulations were evaluated in a laboratory flow-reactor when fresh, after 25,000 simulated miles on a pulse-flame reactor and after 100 or 200 hours of accelerated AMA dynamometer durability. A comparison was made of the effects of contaminant levels on the performance of pulsator - and dynamometer-aged selected catalysts. The 4-fold decrease in contaminant (lead and phosphorus) levels in 76/77 certification fuel compared with the 75/76 fuel significantly improved the durability of 3-way catalysts. The problems of increased NH3 formation on pulsator - and dynamometer-aged catalysts which contain base-metal oxides as oxygen-storage or water-gas shift components is attributed to S-poisoning. An inverse relationship between NH3 formation and the amount of rhodium on aged 3-way catalysts was noted.