A Mg-23.5wt%Ni-5wt%Cu alloy was synthesized by the gravity casting method in a large quantity (7.5 kg). From this alloy, Mg-23.5wt%Ni-xwt%Cu (x = 2.5, 5 and 7.5) samples for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The samples were ground under H(2) in order to obtain a fine powder. These alloys contained crystalline Mg and Mg(2)Ni phases. The Mg-23.5Ni-2.5Cu alloy had the highest hydriding and dehydriding rates after activation among these alloys. The dehydriding curve under 1.0 bar H(2) at 573 K exhibits two stages; the dehydriding rate is high for about 2.5 mm (the decomposition of Mg(2)Ni hydride and Mg hydride in small particles), and then it becomes lower (the decomposition of Mg hydride). (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.