The phase relations and hydrogen storage properties of the (Ca(2-x)Mg(x))Ni(7) alloys were investigated. It was found that the maximum solid solubility of Mg in the (Ca,Mg)(2)Ni(7) phase is about x = 0.5 in the present study. The 'inter-block-layer' type stacking faults exist in the (Ca,Mg)(2)Ni(7) phase when Mg content is very low. However, the density of stacking faults decreases and the lattice parameters reduce as Mg content increases to its maximum solid solubility. Thus the (Ca(1.5)Mg(0.5))Ni(7) alloy has a good reversibility of hydrogen absorption-desorption. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.