Thermal decomposition of püre and mixed copper and manganese carbonate 1.0: 0.225, 1.0: 0.50 and 1.0: 1.0 with respect to CuO: MnO2 was studied. The products at different tem- peratures were characterized by x-ray diffraction analysis. The catalytic activity of the thermal products of püre and mixed oxides was tested in H2O2 decomposition. Thermal treatment of copper carbonate produced CuO at temperature as low as 200°C the increase of temperature of treatment increased the degree of crystallinity of CuO. Manganese carbonate starts to decompose at temperatures higher than 300°C yielding MnO2 whereas at 500°C a mixture of cubic and tetragonal Mn2O3 was detected. At temperatures higher than 500°C, the majör crystalline form of Mn2O3 was cubic. On the other hand, Cu-Mn-mixed carbonates produced only MnC03 crystalline form at low temperatures. At 500°C CuO and other compounds such as Mn2O3 or CuMn2O4 were detected depending on the composition of mixtures. The increase of temperature of treatment was found to be accompanied by an increase in the degree of crystallinity of CuMn2O4. The catalytic activity of these solids in hydrogen peroxide decomposition was found to increase as the activation temperature was increased passing through a maximum when the cata- lysts were preheated at 500°C. The increase of calcination temperature higher than 500°C was accompanined by a marked decrease in the activity in H2O2 decomposition. This may be attri- buted to sintering and / or formation of inactive form of CuMn204.