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. Finally, the steady-state and FTP HC performance of the catalysts were related to the O2 sensor indexes in order to assess the ability of the dual oxygen sensor technique to properly diagnose the HC performance of the catalyst.For the catalyst formulation evaluated in this study, the emission performance reached a maximum with a ceria content of 40%. Higher levels of performance were not achieved by further increasing the ceria content. A catalyst volume of 2.8 L was more effective than a volume of 1.4 L, both for emission performance and for the dual oxygen sensor evaluation. The steady-state HC conversion of the catalyst correlated poorly with the O2 sensor index, as very high HC activity was observed even when the oxygen storage capacity of the catalyst was degraded enough to result in a high (i.e., poor) O2 index. In general, catalysts with high O2 sensor indexes had slightly higher FTP HC emissions than catalysts with low O2 sensor indexes. This results primarily from higher HC emissions during the lightoff cycles 2 and 19 and during the accelerations of the warmed-up cycles. However, different catalysts with similar FTP HC emissions had a wide range of O2 sensor indexes, indicating the low sensitivity of the index to FTP HC emissions. This low sensitivity, coupled with the variability of the O2 sensor index, could make it difficult to reliably diagnose the FTP HC performance of underfloor catalytic converters using dual oxygen sensors.