The phase relations and electrochemical properties of the Ca1-xMgx (Ni0.9Al0.1)(2) alloys have been studied. It was found that the single C15 phase and the single C36 phase exist, respectively, in the approximate composition ranges of x = 0-0.63 and 0.76-1. Between the single-phase regions, the C15 and C36 phases coexist. The hydrogen storage capacity of the C15-type Ca1-xMgx (Ni0.9Al0.1)(2) alloys was lifted with increasing Mg content x. When x=0.6, the hydrogen absorption and desorption capacities are 1.33 and 0.8 wt%, respectively. The alloy electrodes do not need activation during electrochemical measurements. The discharge capacity of the Ca0.4Mg0.6 (Ni0.9Al0.01)(2) alloy is 401 mAh/g at a discharge current of 10 mA/g. However, the cyclic stability should be improved significantly. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.