The activity and durability of the catalysts prepared by the oxidation-reduction treatment of amorphous Co-15 at% Zr, Ni-40 at% Zr and Ni-30 at% Zr-10 at% Sm alloys have been investigated for simultaneous methanation of carbon monoxide and carbon dioxide, It has been found that the Ni-30Zr-10Sm catalyst shows the highest activity among the catalysts examined, and the activity of the Co-15Zr catalyst is lower than those of the nickel-based catalysts, in agreement with the activity for the solo methanation of carbon dioxide. On all the catalysts, carbon monoxide reacts preferentially with hydrogen, and is completely converted into methane at and above 523 K. The remaining hydrogen further reacts with carbon dioxide to form methane. The methanation rate in the H-2-CO-CO2 mixed gas is higher than that in H-2-CO2 mixed gas without CO. This is probably related to the prevention of the formation of surface carbon by the disproportionation reaction of carbon monoxide due to the presence of carbon dioxide. The activity of the Ni-40Zr catalyst at 573 K gradually decreases with reaction time. It has been found that tetragonal ZrO2, the presence of which is responsible for the high activity, is transformed to thermodynamically more stable monoclinic ZrO2 during the catalytic reaction. In contrast to the Ni-40Zr catalyst, the Ni-30Zr-10Sm catalyst sustains the initial high activity, and no structural change is observed during the durability test regardless of the presence of a small amount of hydrogen sulfide.