Microstructoral and hydrogen storage properties of three nanocrystalline melt-spun Mg-base alloys (Mg(90)Cu(2.5)Ni(2.5)Y(5), Mg(85)Cu(5)Ni(5)Y(5) and Mg(80)Cu(5)Ni(5)Y(10)) have been investigated in view of their application as reversible hydrogen storage materials. The activation procedure and the hydrogen sorption kinetics of these alloys were studied by thermogravimetry at different temperatures in the range from 100 C to 380 degrees C. It has been found that these alloys can reach reversible gravimetric hydrogen storage densities of up to 4.8 wt.%-H(2). Even at a low temperature of 100 degrees C, the hydrogenation kinetics of the investigated alloys is rather high in the range of 1.5 wt.%-H(2) per hour. In the hydrogenated state, these alloys consist of MgH(2), high temperature Mg(2)NiH(4), Mg(2)NiH(0.3), YH(2), YH(3) as well as MgCu(2). The presence of MgCu(2) indicates the reaction of Mg(2)Cu with hydrogen. After repeated hydrogenation/dehydrogenation the preservation of a nanocrystalline grain structure has been confirmed by scanning electron microscopy, energy-filtered and conventional transmission electron microscopy. Additionally, the distribution of hydrogen in the hydrogenated sample was mapped by means of electron energy loss spectroscopy. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.