The adsorption and reaction properties of Rh particles supported on a CeO2(100) surface and on polycrystalline CeO2 films annealed at low (LT, 970 K) and high (HT, 1720 K) temperatures were studied using temperature programmed desorption (TPD) and steady-state, CO oxidation kinetics. SEM and XRD of the polycrystalline samples showed significant growth of the ceria crystallite size and change in the surface morphology upon high-temperature annealing. In TPD, a substantial fraction of the CO adsorbed on Rh supported on the LT polycrystalline film was found to desorb as CO2, with the oxygen coming from the CeO2 support. In contrast, oxidation of CO to CO2 was not observed in significant amounts during TPD from Rh on the CeO2(100) sample or the HT film. In agreement with the adsorption measurements, steady-state CO oxidation exhibited a second, ceria-mediated process for Rh on the LT film which was not observed on the HT film. These results indicate that the structure of ceria plays an important role in the reactivity of Rh/ceria catalysts.