The activity of the various CuO species found in supported copper catalysts and the effect of the presence of reaction products, CO2 and H2O, was studied during the complete oxidation of methane. Series of copper catalysts supported on ZrO2, Al2O3 and SiO2 with different metal concentrations were analyzed under identical experimental conditions of reactant concentration and temperature. The catalysts were characterized by TPR, UV-vis spectroscopy and XRD. The results show that the activity of supported CuO is closely related to the kind of Cu species formed on the different supports. It was found that the Cu species formed on ZrO2 and Al2O3 are dependent on the metal loading/support's surface area ratio, and that the activity of highly dispersed Cu is substantially higher than that of bulk CuO. In the case of silica, only the formation of bulk CuO was detected, accounting for the low activity of CuO/SiO2 catalysts. The activity of highly dispersed Cu species formed on ZrO2 is higher than those formed over Al2O3, and it is not significantly affected by the formation of bulk CuO on the surface. On the contrary, the activity of copper species formed on alumina decreases continuously as the Cu loading is increased. Thus, for the range of copper loading studied in this work, the activity of the catalysts, per gram of loaded Cu, follows the sequence: CuO/ZrO2 > CuO/Al2O3 >> CuO/SiO2. It was also found that CO2 does not inhibits the activity of the CuO/ZrO2 catalysts, while water inhibits the combustion reaction of methane, with an estimated reaction order of about -0.2 for temperatures between 360 degrees C and 420 degrees C.) 2007 Elsevier B.V. All rights reserved.