The oxygen-storage properties of ceria in three-way automotive catalysts are promoted and stabilized by mixing with zirconia. In the present study, this promotion was investigated using model catalysts in which ceria lilms were vapor deposited onto alpha-Al2O3, polycrystalline ZrO2, polycrystalline Y2O3-stabilized ZrO2 (YSZ), and YSZ(1 0 0), (1 1 1), and (1 1 0) single crystals. Following deposition of Pd, both TPD of CO and steady-state CO-oxidation kinetics suggest that the ceria films on the zirconia-based substrates were much more easily reduced than films on alpha-Al2O3. Polycrystalline zirconia and YSZ and the YSZ single crystals were equally effective in promoting ceria reducibility. Structural studies of ceria on YSZ(1 0 0), using both TEM and EDSXD (energy-dispersive, surface X-ray diffraction), demonstrate that ceria forms ordered overlayers on YSZ(1 0 0), oriented with respect to the YSZ surface. The lattice parameter for ceria is decreased by only 0.6% compared to bulk CeO2, but the coherence length suggests that the overlayer may have a high defect density. It is suggested that the structure-directing properties of zirconia are responsible for the enhanced properties of ceria-zirconia mixed oxides.