Both laboratory and engine dynamometer testing were used to characterize the relative activity of Pt, Pd and Rh supported on Ce and/or La stabilized supports. In the laboratory studies performance was measured after laboratory aging under conditions designed to simulate severe engine aging. The impact of Pt-Rh and Pd-Rh alloying on performance was examined as well as the cumulative effect of both metals on overall activity. The performance of laboratory aged non-alloyed Pt-Rh and Pd-Rh catalysts was dominated by the Rh function. For Pt-Rh the overall performance features for CO and NOx conversion were very similar over the Rh-only, Pt + Rh (separated metals) and alloyed Pt-Rh catalysts. Pt-Rh alloying was found to have a detrimental impact on high temperature HC performance. In contrast to the Pt/Rh system, alloying between Pd and Rh resulted in performance features more like Pd-only and alloying did not adversely impact high temperature HC performance but instead had a large negative impact on NOx performance. Overall, no benefits to alloy formation were observed and optimum performance required separation of the metals in either case. Complementary engine dynamometer aging and testing studies confirmed the improved performance of Pt/Rh and Pd/Rh catalysts using preparation strategies designed to separate the metals.