Reaction rates and activation energies were measured for carbon monoxide oxidation over thin films of metallic copper, copper (I) oxide, and copper (II) oxide grown on graphite. The reactions were carried out in the temperature range 200-350-degrees-C at total pressures of 100 Torr. Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) were used to characterize the growth and oxidation states of the copper films before and after catalytic reactions. The stability of a given oxidation state of copper under reaction conditions was found to be a function of the oxidizing power of the CO/02 partial pressure ratio : metallic copper, Cu2O, and CuO were stable at 97/3, 90/10, and 66/33 ratios, respectively. The rate of reaction at 300-degrees-C decreased with increasing copper oxidation state (Cu > Cu2O > CuO) and the activation energy increased with increasing copper oxidation state (Cu 9 < Cu2O 14 < CuO 17 kcal/mol). The kinetic parameters for CO oxidation over platinum foil was tested at a 66/33 CO/02 ratio for comparison. The reaction rate over platinum was approximately equal to that over metallic copper at 300-degrees-C and had a larger activation energy (20 kcal/mol) than CuO. The possible mechanisms for CO oxidation over the different oxidation states of copper are discussed.