The reactivity of the H2-activated Ni(II)- and Co(II)-bearing ferrites with different levels of metal substitution have been studied for CO2 → C decomposition in comparison with that of the H2-activated magnetite. Ni 2+ and Co2+ have been substituted for Fe2+ or Fe3+ in magnetite with the spinel-type structure up to 14 % and 26 % of the mole ratio of Ni2+ and Co2 + to the total Fe content respectively. The reactivity of the Ni(II)- and Co(II)-bearing ferrite increased with the level of metal substitution. Especially, the Ni(II) substitution significantly facilitated the CO2 → C decomposition in a batch system. The rates of H2-activation (reduction) and CO2-decomposition (oxidation) for N(II)-bearing ferrite were studied by a thermogravimeteric analysis. The rates of both H2-activation and CO2-decomposition were much improved in the Ni(II)-bearing ferrite with the Ni(II)/Fetoul mole ratio of 14%. It is considered that the reduced Ni(II) ions which were formed on the surface of the ferrite is very active to facilitate both dissociation reactions of 1) H2 → 2 Hads and 2) CO2→ Cads + 2Oads,. From the change in the lattice constant of the Ni(II)-bearing ferrite during the H2-activation and CO2-decomposition, the oxygens in CO2 were considered to be incorporated into the oxygen deficient spinel lattice of the oxygen deficient Ni(II)-bearing ferrite which had been formed by the H2-activation.