In order to improve the cycle stability of La - Mg - Ni system (A(2)B(7)-type) alloy electrode, a small amount of Co was added in the La0.75Mg0.25Ni3.5 alloy. The effects of Co content on the microstructures and electrochemical performances of the alloys were investigated in detail. The results by XRD and SEM show that the alloys have a multiphase structure which is composed of the LaNi5, (La, Mg)(2)Ni-7 two major phases and a small amount of the LaNi2 phase. The cell volumes of the LaNi5 and (La, Mg)(2)Ni-7 phases enlarge with the increasing Co content in the alloys. With the increasing Co content, some electrochemical properties and kinetic parameters of the alloy, involving the discharge capacity, high-rate discharge ability (HRD), the polarization resistance (R-p), the loss angle (psi) and the limiting current density (I-L) first increase and then decrease. The addition of Co slightly improves the cycle stabilities of the alloy electrodes. The mechanism of the efficiency loss of the experimental alloy was investigated by means of SEM and X-ray photoelectron spectroscopy (XPS). The results indicate that the fundamental reasons for the capacity decay of the La0.75Mg0.25Ni3.5Cox (x=0, 0.2, 0.4, 0.6) alloy electrodes are the pulverization of the alloy particle and corrosion/oxidation of La and Mg in alkaline electrolyte. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.