The morphology and reducibility of vapor-deposited ceria films supported on yttria-stabilized zirconia (100) (YSZ(100)) and α-Al2O3(0001) single crystals were studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The results of this study show that the gas environment has a significant effect on the structure of the ceria films on both substrates. CeO2 films on α-Al2O3(0001) were found to be stable in a reducing environment at temperatures up to 1000K, but underwent agglomeration and reaction with the support to form CeAlO3 upon annealing at 1273 K in air. Heating CeO2/YSZ(100) in air at 1273 K caused the ceria thin film to agglomerate into bar-shaped features which were re-dispersed by subsequent annealing in vacuum. Interactions at the CeO2-YSZ interface were also found to dramatically enhance the reducibility of ceria films supported on YSZ(100).