The effect of Mn content on the crystal structure and electrochemical characteristics of La0.7Mg0.3Ni2.975-XCo0.525Mnx (x = 0, 0.1, 0.2, 0.3, 0.4) alloys has been studied systematically. The results of the Rietveld analyses show that all these alloys mainly consist of two phases: the La(La,Mg)(2)Ni-9 phase with the rhombohedral PuNi3-type structure and the LaNi5 phase with the hexagonal CaCu5-type structure. The pressure-composition isotherms shows that the partial substitution of Mn for Ni results in lower desorption plateau pressure and steeper pressure plateau of the alloy electrodes. For a Mn content of x = 0.3, the electrochemical performances, including specific discharge capacity, high rate chargeability (HRC) and high rate dischargeability (HRD), of the alloy are preferable. Moreover, the data of the polarization resistance R-p and the exchange current density I-0 of the alloy electrodes is consistent with the results of HRC and HRD. The hydrogen diffusion coefficient D increases with increasing Mn content, and thereafter increases the low temperature dischargeability (LTD) of the alloy electrodes. © 2004 Elsevier Ltd. All rights reserved.