CO oxidation was measured at atmospheric pressure over copper oxide catalysts supported on gamma-Al2O3, 3% ZrO2 + gamma-Al2O3, ZrO2 (UFP) and ZrO2, and their catalytic action was investigated by X-ray diffraction (XRD), temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD). Copper oxide supported on all these catalysts existed as an excellent dispersed state; at the moment, the catalytic activity was the highest. Copper in small amounts had a strong tendency to associate with zirconium oxide and caused a large increase in activity. When the zirconium oxide surface was saturated with copper, excess copper formed bulk copper oxide particles contributing little to the catalytic activity. In the TPR experiments, three TPR peaks over CuO/ZrO2 (UFP) and CuO/ZrO2 were observed at 170-210 degrees C (peak alpha and beta) and above 210 degrees C (peak beta). In conjunction with the observations by XRD, it was concluded that highly dispersed copper(II) oxide and bulky copper(II) oxide were reduced to metallic copper, giving peaks alpha, beta and gamma, respectively. The TPD analyses showed CuO/ZrO2 catalysts exhibited a TPD peak of surface’s adsorbed oxygen below 350 degrees C. Thus, not only the presence of surface’s adsorbed oxygen but also the desorptibility of al oxygen species from copper oxide located on ZrO2, which increased with CuO content, may also be related to the catalytic activity.