In order to investigate the influences of the stoichiometric ratios of B/A (A: gross A-site elements, B: gross B-site elements) and the substitution of Co for Ni on the structure and the electrochemical performances of the AB(2.5-3.5)-type electrode alloys, the La-Mg-Ni-Co system La0.75Mg0.25Ni2.5Mx (M = Ni, Co; x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) alloys were prepared by induction melting in a helium atmosphere. The structures and electrochemical performances of the alloys were systemically measured. The obtained results show that the structures and electrochemical performances of the alloys are closely relevant to the M content. All the alloys exhibit a multiphase structure, including LaNi2, (La, Mg)Ni-3 and LaNi5 phases, and the major phase in the alloys changes from LaNi2 to (La, Mg)Ni-3 + LaNi5 with the variety of M content. The electrochemical performances of the alloys, involving the discharge capacity, the high rate discharge (HRD) ability, the activation capability and the discharge potential characteristics, significantly improve with increasing M content. When M content x increases from 0 to 1.0, the discharge capacity rises from 177.7 to 343.62 mAh/g for the alloy (M = Ni), and from 177.7 to 388.7 mAh/g for the alloy (M = Co). The cycle stability of the alloy first mounts up then declines with growing M content. The substitution of Co for Ni significantly ameliorates the electrochemical performances. For a fixed M content (x = 1.0), the substitution of Co for Ni enhances the discharge capacity from 343.62 to 388.7 mAh/g, and the capacity retention ratio (S-100) after 100 charging-discharging cycles from 51.45% to 61.1%. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.