A three-way catalyst containing only Rh and another containing Pt, Pd, and Rh were aged in engine exhaust. The engine was operated on fuel containing phosphorus or lead additives at concentrations high enough to accelerate the poisoning of the catalysts. Lead did not noticeably affect either catalyst's CO conversion. It did, however, reduce both catalysts' HC and NO conversions. The poisoning effect of phosphorus increased as the amount of phosphorus on the catalyst increased. While the amounts of phosphorus in the fuel consumed by the engine were similar in each test, the phosphorus collected by the catalysts varied greatly, depending on the nature of the fuel additives.