The adsorption and reaction of CO on Rh particles supported on oxidized and partially reduced epitaxial ceria films on yttria-stabilized cubic zirconia (YSZ) (100), (110), and (111) single crystals was studied using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The ceria films on YSZ were highly reducible, reacting with CO adsorbed on Rh particles to produce CO2 during TPD. After reduction of the ceria, however, the CO TPD results contained two features : a low-temperature state resulting from desorption of molecular CO and a high-temperature state resulting from the recombination of C and O atoms produced by CO dissociation at lower temperature. The fraction of CO which dissociated on YSZ-supported Rh/ceria increased with the extent of reduction of the ceria surface. TPD studies with isotopically labeled CO demonstrated that the O atoms produced by CO dissociation undergo exchange with oxygen from the ceria lattice. The CO dissociation reaction was found to be metal specific and was not observed for either Pd or Pt supported on reduced ceria/YSZ.