The adsorption and reaction of CO on Rh-loaded ceria was studied by soft x-ray photoelectron spectroscopy. The system was studied as a function of the degree of oxidation of the ceria and different preparations of the Rh. Molecular CO and atomic C on Rh are readily distinguished in the C Is spectra. CO adsorbed directly on ceria, possibly as carbonate or carboxylate, is also observed in the C Is spectra. The degree of dissociation of CO an Rh deposited on ceria (Rh/CeOx) is directly dependent on the degree of oxidation of the ceria. Nearly total decomposition can occur if the ceria substrate is highly reduced. Molecular CO also persists to a higher temperature on Rh deposited on reduced ceria compared with Rh an oxidized ceria. The degree of dissociation does not depend on the amount of Rh deposited or on the annealing treatment after deposition.