Mg-oxide and Mg-oxide-Ni hydrogen storage alloys were prepared by mechanical grinding under H-2 (reactive mechanical grinding). Among these alloys, Mg-(7.5wt%Fe2O3, 7.5wt%Ni) prepared by grinding for 4h with nano-structured Fe2O3 particles and Ni showed the best hydrogen storage properties. The as-milled sample absorbed 4.75 wt% H at 593 K under 12 bar H-2 for 60 min. Its activation was accomplished after two hydriding-dehydriding cycles. The activated sample absorbed 4.36 wt% H at 593 K under 12 bar H-2 for 60 min and desorbed 4.26 wt% H at 593K under 1.0 bar H-2 for 60 min. After hydriding-dehydriding cycling, Mg2Ni, Fe and MgO are formed. The large bar-like hydrided particles have round and expanding surfaces, suggesting that they expanded during the hydriding reaction, while the dehydrided particles exhibit contracted surfaces with wrinkles. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.