Ml(1-x)Mg(x)Ni(2.4)Co(0.6) (x=0-0.6) hydrogen storage alloys were prepared by inductive melting and the effect of Mg on the crystal structure and hydrogen storage characteristics was investigated. These alloys are confirmed to be LaNi3 phase with the PuNi3 type structure. The Mg substitution for Ml in the alloys leads to a shrinkage of the unit cell volume and a reduction of the stability of alloy hydrides. With increasing the Mg substitution for Ml, the atom ratio of hydrogen to alloy (HIM) decreases, but the weight ratio of HIM and the discharge capacity of electrodes increase in a certain range of Mg content (alpha <= 0.4). The Ml(0.6)Mg(0.4)Ni(2.4)Co(0.6) alloy has the highest weight ratio of H/M (1.75 wt.%) at 1 Wa H-2 and 298 K and the largest discharge capacity of electrodes (326 mAh g(-1)). In addition, the increase of Mg substitution for Ml results in better high-rate dischargeability and cycling stability of electrodes. (c) 2005 Elsevier B.V. All rights reserved.