The adsorption and reaction of CO on Rh particles supported on stoichiometric and partially reduced CeO2(111) surfaces was studied using a combination of HREELS and TPD. A fraction of the CO adsorbed on the supported Rh particles was found to undergo dissociation to produce adsorbed C and O atoms. TPD results for isotopically labeled CO demonstrated that O atoms produced by CO dissociation rapidly exchange with the oxygen in the ceria lattice. The fraction of adsorbed CO which dissociated was found to increase significantly with the extent of reduction of the CeO2(111) surface, suggesting that oxygen vacancies on the surface of the support play a direct role in the CO dissociation reaction.