We tried to improve the H-2-sorption properties of Mg by mechanical grinding under H-2 (reactive mechanical grinding) at various weight ratios of sample to ball (1/9, 1/27 and 1/45) with 10 wt% Fe2O3. The revolution speed was 250 rpm and the milling time was 2 h. The sample Mg-10 wt% Fe2O3, prepared by milling at the weight ratio of sample to ball 1/45, has the best hydrogen-storage properties. It absorbs 5.56 wt% hydrogen at the first cycle at 593K under 12 bar H-2 for 60 min. Its activation is accomplished after two hydriding-dehydriding cycles. The activated sample absorbs 4.26 wt% hydrogen at 593 K, 12 bar H-2 for 10 min. The reactive grinding of Mg with Fe2O3 increases the H-2-sorption rates by facilitating nucleation (by creating defects on the surface of the Mg particles and by the additive), by making cracks on the surface of Mg particles and reducing the particle size of Mg and thus by shortening the diffusion distances of hydrogen atoms. Hydriding-dehydriding cycling also increases the H-2-sorption rates by creating defects on the surface of the Mg particles, and by making cracks on the surface of Mg particles and reducing the particle size of Mg. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.