Abstract Repetitive hydriding-dehydriding cycles of MmNi 4.15Fe 0.85 in hydrogen containing small amounts of oxygen as a minor impurity were investigated in order to study the extrinsic degradation of the compound. The initial stage of cycles exhibited a pronounced degradation of the hydrogen absorption capacity which was attributed to the preferential adsorption of oxygen on the active sites for the dissociative chemisorption of hydrogen. However, the hydrogen absorption capacity was gradually recovered by the subsequent cycling or by heat treatment. This may be due to the separation of MmNi 4.15Fe 0.85 to the Mm-oxide and elemental nickel or iron by either pressure cycling or heat treatment. The recovery of the effective hydrogen absorption capacity occurred much sooner at 30°C than 0°C. This may be mainly due to the difference of the amount of adsorbed oxygen on the alloy surface and the mobility of the diffusional surface rearrangement with temperature. In particular, the hydrogenation properties of MmNi 4.15Fe 0.85 metal hydride were greatly improved by the heat treatment. It is suggested that the improvement of hydriding properties of MmNi 4.15Fe 0.85 by the heat treatment is mainly due to the catalytic effect of metallic nickel or iron segregated at the surface. The fact that surface segregation was enhanced by heat treatment was confirmed through the measurement of magnetization properties of the compound.