In this paper, the effect of the compositions on the structural and electrochemical characteristics of the La0.7Mg0.3 (Ni0.85Co0.15)(x) (x = 3.0,3.5,4.0,5.0) hydrogen storage alloys have been investigated systematically. The results of the XRD Rietveld analyses show that the structures of the alloys change obviously with increasing x from 3.0 to 5.0. The main phase of the alloys with x = 3.0-3.5 is (La, Mg)Ni-3 phase (PuNi3-type structure), but the main phase of the alloys with x = 4.0-5.0 is LaNi5 phase (CaCu5-type structure). Moreover, the phase abundance, lattice parameters and cell volumes of the (La, Mg)Ni3 phase and the LaNi5 phase change with increasing x. The electrochemical studies show that the maximum discharge capacity increases from 355.4 mAh/g (x = 3.0) to 395.6 mAh/g (x = 3.5) and then decreases to 226.8 mAh/g (x = 5.0). As the discharge current density is 1250 mA/g, the high rate dischargeability (HRD) increases from 67% (x = 3.0) to 81.3% (x = 3.5) and then decreases to 74.9% (x = 5.0). Furthermore, the exchange current density (I-0), the limiting current density (I-L) and the diffusion coefficient (D), of the alloy electrodes all first increase and then decrease with increasing x from 3.0 to 5.0, which is consistent with the variation of the HRD. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.