Redox properties of CeO2 and Pt-Rh/CeO2 were studied by temporal analysis of products (TAP) method using alternative pulses of CO and O-2. A portion of pulsed CO was oxidized to CO2 and a portion of CO was adsorbed on the surface. Pulsing O-18(2) onto the catalyst which has surface species derived from CO, evolved CO2 contained no O-18 suggesting that the surface species will be carbonate ions.