When the three-way conversion (TWC) catalysts were aged on representative 1977 unleaded commercial fuel, they had higher conversion efficiencies and improved durability than catalysts aged on modified 1975 FTP specification fuel containing approximately 0.025 g/gal Pb and low levels of phosphorus. As the rhodium content was increased in a series of platinum-rhodium TWC catalysts, the maximum conversion efficiency and durability increased, and the mine recovery ratio of Pt/Rh was found to be most susceptible to lead poisoning. However, good NOx efficiencies can be obtained from mine recovery ratio Pt/Rh TWC catalysts after 25,000 miles of engine aging. It is shown that improvements in catalyst formulations containing mine recovery ratio Pt/Rh resulted in higher conversion efficiencies. Monolithic TWC catalysts require approximately the same total precious metal as current oxidation catalysts. Particulate TWC catalysts show beneficial effects in increasing the precious metal loading above 15 grams/cubic foot.