The cycle durability of Ca–Mg–Ni hydrogen storage alloys, which are investigated in the Japanese national project (International Clean Energy Network), WE-NET, was examined using pure H
2 (99.99999%). The tested alloys were Ca
0.25Mg
0.66Y
0.09Ni
1.86 (cubic MgCu
2-type structure), Ca
0.55Mg
0.45Ni
3 (rhombohedral PuNi
3-type) and Ca
0.5Mg
0.5Ni
2 (MgCu
2-type). Although initially the dynamic hydrogen storage capacities were H/M
0.7, they decreased steeply within a few cycles, and then they declined gradually. At around 400th cycle, they decreased to nearly half of the maximum capacities regardless of crystal structures and compositions of the tested alloys. However, their equilibrium capacities did not decrease as the dynamic ones with progress of hydrogen sorption. As for the bulk, not serious disproportionation but pulverization and lattice expansion were observed by X-ray diffractometry (XRD) and microscopic analyses. The expansion indicated that some amounts of hydrogen remained in the lattice cells. It was considered that pulverization of the alloy particles and amorphous shell formed on the surface prevented the alloys from smooth hydrogen sorption, and which led to the decrease of the dynamic hydrogen storage capacities without that of the equilibrium ones in the cycle tests.