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A study was performed to assess the effects of the catalyst volume and the ceria content in the washcoat on the aged emission performance of underfloor catalytic converters containing platinum and rhodium. Catalyst volumes of 1.4 L and 2.8 L were evaluated, while the ceria level was varied from 0 to 60% of the weight of the washcoat. The concentration of noble metals (g/L) was the same for both catalyst volumes, so the larger volume also contained more noble metal. Catalyst performance was evaluated on an air/fuel ratio sweep test, at steady-state conditions on an engine, and on the FTP test. In light of the new catalyst monitoring requirements for OBD II, each catalyst was also evaluated at steady-state conditions using a dual oxygen sensor technique in order to produce an O2 sensor index. The evaluations were performed at several intermediate stages as the catalysts were aged on engines using high temperature durability schedules intended to simulate high mileage conditions.