The effect of Mg content on the structural characteristics and hydrogen storage properties of the Ca3.0-xMgxNi9 (x = 0.5, 1.0, 1.5 and 2.0) alloys was investigated. The lattice parameters and unit cell volume of the PuNi3-type (Ca, Mg)Ni-3 main phase decreased with increasing Mg content. The 6c site of PuNi3-type structure was occupied by both Ca and Mg atoms. Moreover, the occupation factor of Ca on the 6c site decreased with the increase of Mg content. The hydrogen absorption capacity of the alloys decreased due to higher Mg content. However, the thermodynamic proper-ties of hydrogen absorption and desorption were improved and the plateau pressures were increased. When x = 1.5-2.0, the Ca3.0-xMgxNi9 alloys had favorable enthalpy (Delta H) and entropy (Delta S) of hydride formation. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.