Ultrafine Ni(II) ferrite (UNF) with 36% Ni2+ substitution for Fe2+ in magnetite has been synthesized in order to study its catalytic activity and selectivity toward CO2 methanation at 300 degrees C. Ni(II) ferrites were prepared by two different methods : (1) coprecipitation of Ni2+, Fe2+, and Fe3+ at 60 degrees C followed by heating to 300 degrees C and (2) oxidation of aqueous suspension of Fe2+ and Ni2+ hydroxides at 65 degrees C (oxidation method). The BET surface area of the initial UNF synthesized by coprecipitation was 73 m(2) g(-1), eight times as large as that of the Ni(II) ferrite obtained by the oxidation method (NF; 9 m(2) g(-1)). It rapidly decreased to 26 m(2) g(-1) by sintering in the initial stage of methanation. The yield of CH4 obtained on the UNF catalyst was 1.5-6.0 times larger than that on the NF catalyst in a 2-h run of the methanation, depending on a flow rate of reactant H-2/CO2 mixed gas. It was found that the selectivity for CH4 was much improved in UNF (96%) in comparison with NE XRD and chemical analysis showed that the UNF and NF were transformed to oxygen-deficient ferrite forms during the methanation. The specific activity (mol m(-2) s(-1)) of UNF for CH4 formation was rather lower than that of NF, about half the activity of NF, which could be explained assuming that oxygen sublattice points on the catalyst surface formed by H-2 reduction serve as active sites for methanation. On the other hand, the activity for CO formation significantly decreased on UNF, due to the change in the nature of the active site.