Abstract Hydrogenation properties of the MmNi 4.5 Al 0.5 Zr x ( x = 0−0.2) alloys are investigated by means of PC-isotherm measurements, X-ray diffractometry, scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). As the Zr content increases, the first hydriding reaction is completed in a shorter time, but the plateau pressure is sloped and decreases. At a composition of x = 0.05, a maximum hydrogen storage capacity is obtained with a enhanced activation property. Upon pressure cycling of the MmNi 4.5 Al 0.5 Zr 0.5 alloy, the hydrogen storage capacity is decreased by only 8% after 4300 cycles. From the X-ray diffraction, SEM and EDX tests, new second phases are observed at high Zr content above x = 0.1. The major second phase is confirmed to be the ZrNi 3 phase. It is considered that the improved activation properties of the MmNi 4.5 Al 0.5 Zr x alloys are caused by the strong catalytic effect of ZrNi 3 on the dissociative chemisorption of hydrogen molecule. Among the investigated alloy systems, it is suggested that the most suitable alloy is the MmNi 4.5 Al 0.5 Zr 0.05 as a new hydrogen storage material due to its the large hydrogen storage capacity, the improved activation behavior and the strong resistance to the degradation in the subsequent cyclic operation.