The beneficial effects of CeO2/ZrO2 solid solutions on the performance of fully formulated Pt, Rh TWC (three-way-conversion) catalysts were measured using both stand dynamometer and FTP testing after severe engine aging. The performance advantages were consistent with an enhancement of the chemical promotional effects of CeO2. These included increased effectiveness for CO and NOx conversion and to a lesser extent for HC compared to catalysts prepared with the same loading of Ce and Zr but no solid solution formation. Higher performance could be achieved with the CeO2/ZrO2 solid solution catalysts having half the Ce loading of conventional catalysts prepared with pure CeO2.The physico-chemical properties of the catalysts were characterized using both XRD and TPR. XRD was used to determine the degree of solid solution formation between CeO2 and ZrO2 and TPR was used to characterize the redox properties/oxygen storage of the catalysts before and after aging. Measurable promotional effects were only observed for Zr levels in excess of 18 mole% after severe engine aging. TPR analysis showed that fresh catalysts had strong coupling between the precious metals (PMs) and the CeO2 reduction. However, after severe aging the pure CeO2 containing catalysts lost their oxygen storage properties, but measurable redox activity was observed for the CeO2/ZrO2 solid solution-containing catalysts.