Nanostructured MgH2-Ni/Nb2O5 nanocomposite was synthesized by high-energy mechanical alloying. The effect of MgH2 structure, i.e. crystallite size and lattice strain, and the presence of 0.5 mol% Ni and Nb2O5 on the hydrogen-desorption kinetics was investigated. It is shown that the dehydrogenation temperature of MgH2 decreases from 426 degrees C to 327 degrees C after 4 h mechanical alloying. Here, the average crystallite size and accumulated lattice strain are 20 nm and 0.9%, respectively. Further improvement in the hydrogen desorption is attained in the presence of Ni and Nb2O5, i.e. the dehydrogenation temperature of MgH2/Ni and MgH2/Nb2O5 is measured to be 230 degrees C and 220 degrees C, respectively. Meanwhile, the dehydrogenation starts at 200 degrees C in MgH2-Ni/Nb2O5 system, revealing synergetic effect of Ni and Nb2O5. The mechanism of the catalytic effect is presented. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.