The effect of the substitution of La by Mg in the hypo-stoichiometric La1-xMgxNi3.8Co0.3Mn0.3Al0.4 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) hydride alloys were metallurgically and electrochemically characterized in order to use them as negative electrodes in Ni-MH batteries. The Mg content was added to mother alloys by means of low energy mechanical alloying. The microstructure, composition and crystalline structure of the samples were analyzed by SEM, EDS and X-ray diffraction, respectively, and their electrochemical properties were analyzed by charge/discharge cycling stability and rate capability. The electrochemical results show that the alloy LaNi3.8Co0.3Mn0.3Al0.4 reaches a maximum discharge capacity of 181.7 mAh/g. La0.85Mg0.15Ni3.8Co0.3Mn0.3Al0.4 presents the best behavior in electrochemical cycling stability with a capacity decay of 4.4% after 50 cycles and a capacity reduction rate of 0.117 mAh/g per cycle. The alloy La0.8Mg0.20Ni3.8Co0.3Mn0.3Al0.4 obtained the highest discharge capacity at high rate discharge. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.