Variations in the oxidation state of copper during CO catalytic oxidation over powdered CuO and Cu 2 O in a gas mixture of 5% CO/He and air (O 2 ) were investigated by means of an appropriate combination of evolved gas analysis, thermogravimetry, XRD, and FT-IR in the temperature range of RT to 350°C, and these variations were compared with those observed when both oxides were individually reacted with CO (5% CO/He) and with air. The catalytic rate over CuO was found to almost agree with the bulk reduction rate of CuO→Cu, while the catalytic rate over Cu 2 O agreed with the bulk reduction rate of CuO*→Cu 2 O, where CuO* denotes a metastable divalent copper oxide produced in the oxidizing process of Cu 2 O. Adsorption of O 2 − and/or O − was characteristic of CuO*, in contrast with that of O 2 and/or O of CuO. Variations in copper valence of a catalyst surface were concluded to be II ⇄ I for CuO and I→II ⇄ I for Cu 2 O, and the catalytic rates were determined by the reducing processes of CuO and CuO* given by O 2− lc +O 2 ads (or O ads )+CO ads →CO 2 +O 2 ads (or O ads )+2e − followed by copper valence changes of II→0 and O 2− lc +O − 2 ads (or O − ads )+1/2CO→1/2O 2− lc +O − 2 ads (or O − ads )+1/2CO 2 +e − followed by that of II→I, where O 2− lc denotes lattice oxygen.