The relative rates of the CO+oxygen storage material (OSM)double right arrow CO2 reactlion (R-CO2) and the amounts of rate enhancement obtained upon Pt promotion were examined for ceria and ceria-zirconia OSM. The effect of Pt surface area on R-CO2 was decoupled from metal-oxide surface area by pre-sintering the oxides prior to Pt deposition. We find that R-CO2 is linearly dependent on Pt area over Pt/CeO2, but over Pt/Ce0.75Zr0.25O2, the rate is independent of Pt surface area above a threshold surface area. Although Pt sinters more readily on the Ce0.75Zr0.25O2 support, the dispersion effect is more than compensated by the enhanced availability of "bulk" O2- within the Ce0.75Zr0.25O2 particles, Furthermore, this study demonstrates that, on a unit surface area basis, the ceria-zirconia support is at least two times more active for the oxygen release reaction than the pure ceria support when the materials are slightly reduced. It is also shown that O2-diffusion is not rate-limiting for the ceria reduction reaction when the reaction is carried out in the kinetically limited regime at 500 degrees C with CO as the reductant.