The catalyst performance and structure of Pt and Rh catalyst systems are studied for fresh catalysts and samples aged to mimic in-use catalyst performance. It was observed that both fresh Pt and Rh catalysts are highly active for three-way conversions of HC/CO/NO. However, after exposure to exhaust at elevated temperature, Pt and Rh catalysts deteriorated severely, and only the Pt-Rh catalyst remains highly active for the three-way conversions. In order to understand this observed synergism between Pt and Ph in the Pt-Ph catalyst, AEM, XPS, and XRD were used to characterize catalyst structural change and establish a correlation between catalyst performance and structure. Pt catalysts deactivate at high temperature under reducing conditions and more severely under oxidizing conditions. Ph catalysts deactivate under oxidizing conditions by forming Ph-aluminate species. The regeneration of these Rh-aluminate species to highly active Ph metal particles under reducing conditions take place at elevated temperature, e.g., 1173 K, which is significantly higher than normal operating temperature (673 K). For the Pt-Rh catalyst, Pt and Ph go through deactivation cycles the same as that for the individual Pt and Rh catalysts in the simulated aging process. However, Ph in the aged Pt-Ph catalyst can regenerate in exhaust at temperatures as low as 560 K, which is significantly lower than the normal operating temperature. This is attributed to H-2/CO spillover on Pt particles contained in the Pt-Ph catalyst which facilitate the regeneration of the inert Ph-aluminate species to active Rh metal particles.