Activated magnetite (Fe3O4-δ) has the capability of decomposing CO2 proportional to the δ-value at comparatively low temperature of 300 oC. To enhance the CO2 decomposition capability of Fe3O4-δ, (Fe1-xCox)3O4-δ and (Fe1-xMnx)3O4-δ were synthesized and then reacted with CO2. Fe1-xCoxC2O4·2H2O powders having Fe to Co mixing ratios of 9:1, 8:2, 7:3, 6:4, and 5:5 were synthesized by co-precipitation of FeSO4·7H2O and CoSO4·7H2O solutions with a (NH4)2C2O4·H2O solution. The same method was used to synthesize Fe1-xMnxC2O4·2H2O powders having Fe to Mn mixing ratios of 9:1, 8:2, 7:3, 6:4, 5:5 with a MnSO4·4H2O solution. The thermal decomposition of synthesized Fe1-xCoxC2O4·2H2O and Fe1-xMnxC2O4·2H2O was analyzed in an Ar atmosphere with TG/DTA. The synthesized powders were heat-treated for 3 hours in an Ar atmosphere at 450 oC to produce activated powders of (Fe1-xCox)3O4-δ and (Fe1-xMnx)3O4-δ. The activated powders were reacted with a mixed gas (Ar : 85 %, CO2 : 15 %) at 300 oC for 12 hours. The exhaust gas was analyzed for CO2 with a CO2 gas analyzer. The decomposition of CO2 was estimated by measuring CO2 content in the exhaust gas after the reaction with CO2. For (Fe1-xMnx)3O4-δ, the amount of Mn2+ oxidized to Mn3+ increased as x increased. The δ value and CO2 decomposition efficiency decreased as x increased. When the δ value was below 0.641, CO2 was not decomposed. For (Fe1-xCox)3O4-δ, the δ value and CO2 decomposition efficiency increased as x increased. At a δ value of 0.857, an active state was maintained even after 12 hours of reaction and the amount of decomposed CO2 was 52.844 cm3 per 1 g of (Fe0.5Co0.5)3O4-δ.