In this paper, the effects of both Co substitution and annealing treatment on the structures and electrochemical properties of La0.7Mg0.3Ni2.45-xCo0.75+xMn0.1Al0.2 (x =0.00, 0.15, 0.30) hydrogen storage alloys have been studied. X-ray diffraction (XRD) analyses show that the main phases of all of the alloys are (La, Mg)Ni-3 phase (PuNi3-type structure) and LaNi5 phase (CaCu5-type structure). Electrochemical investigations show that increasing Co content and annealing treatment can considerably enhance the cyclic stability of the alloy electrodes. For La0.7Mg0.3Ni2.15Co1.05Mn0.1Al0.2 alloy, the value of C-100/C-max was only 65.5% at its as-cast, however, the value reached 80.5% after an annealing treatment of 1173 K x 8 h. The maximum discharge capacity of the alloy electrodes increased by annealing treatment and decreased slightly when Co content increased from x = 0.00 to 0.30 at its as-cast state. But the exchange current density (I-0), the limiting current density (I-L) and the diffusion coefficient of hydrogen (D) of the alloy electrodes decreased, leading to a corresponding reduction of the high rate dischargeability (HRD), with increasing Co content and annealing treatment. (C) 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy.