To reduce CO2 gas, high specific surface and high activity of (MnxZn1-x)Fe2O4 and (MnxNi1-x)Fe2O4 nano-sized powders were synthesized by the hydrothermal process and used as catalysts. Effects of the composition of synthesized ferrites, the activated time of the catalysts, and the flow rate of carbon dioxide on the decomposition of carbon dioxide were investigated. The crystalline Mn-Zn ferrite catalysts with 21-29 nm nano-scale particle size and 73-143 m(2) g(-1) specific surface area were synthesized by hydrothermal process at 150degreesC for 2 h. The (Mn0.33Zn0.67)Fe2O4 catalyst shows the best CO2 decomposition performance after reduction in H-2 at 300degreesC for 4 h. The crystalline Mn-Ni ferrite catalysts with 18-24 nm nano-scale particle size and 107-153 m(2) g(-1) specific surface area were synthesized by hydrothermal process at 140degreesC for 2 h. The (Mn0.67Ni0.33)Fe2O4 catalyst shows the best CO2 decomposition performance after reduction in H-2 300 degreesC for 2 h. The Mn-Zn ferrite showed a better CO2 decomposition efficiency than that of Mn-Ni catalyst with the same Mn content. (C) 2004 Elsevier B.V. All rights reserved.