The decomposition of NO on Rh on oxidized and reduced cerium oxide was studied as a function of NO coverage and temperature by soft X-ray photoelectron spectroscopy. The degree of decomposition is increased and the onset temperature for decomposition is reduced when Rh is supported on reduced ceria (Rh/CeOx) compared to Rh on oxidized ceria (Rh/CeO2). When the initial NO coverage was reduced to 35% of saturation the dissociation activity was enhanced on both Rh/CeO2 and Rh/CeOx. However, at any given temperature and coverage, the dissociation activity was greater on Rh/CeOx. O vacancies on the reduced CeOx were poisoned by exposure to H2O at 300 K to produce a hydroxylated surface. The NO dissociation activity for Rh on the hydroxylated CeOx was the same as that on the open surface. These results demonstrate that O spillover and coverage effects are insufficient to explain the enhanced activity of Rh on reduced CeOx.