Mg-Cu alloy nanoparticles were in situ prepared by a physical vapor condensation method (arc discharge) in a mixture of argon and hydrogen. Four crystalline phases, Mg, Mg(2)Cu, MgCu(2) and MgO, were formed simultaneously during the arc-discharge evaporation. Detailed experiments revealed that nanostructured hydrogen-active phases of Mg(2)Cu and Mg exhibit enhanced hydrogen absorption kinetics possibly due to the small grain size and surface defects. The maximal hydrogen storage contents of Mg-Cu alloy nanoparticles can reach 2.05 +/- 0.10 wt% at 623 K. (c) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.