The effects of the substitution of Mn for Ni on the structures and electrochemical characteristics of the as-cast and quenched La0.5Ce0.2Mg0.3Co0.4Ni2.6-xMnx (x=0, 0.1, 0.2, 0.3, 0.4) alloys were studied systematically. The results obtained by XRD, SEM and TEM indicate that the as-cast and quenched alloys mainly consist of two major phases the (La, Mg)Ni-3 and LaNi5 and a residual phase LaNi. With increasing Mn content in the alloys, the abundance of the (La, Mg)Ni-3 phase decreases and that of the LaNi5 phase increases and both the lattice parameters and the cell volumes of major phases (La, Mg)Ni-3 and LaNi5 increase. The as-quenched alloys display a nanocrystalline structure, and the grain sizes of the alloys are in a range of 20-30 nm. The results by the electrochemical measurements indicate that the discharge capacities of the as-cast and quenched alloys first increase and then decrease with the variety of Mn content and obtain the maximum values for a Mn content of x = 0.2. The substitution of Mn for Ni clearly improves the high rate discharge abilities (HRDs) and the activation capabilities of the as-cast and quenched alloys, but it slightly impairs the cycle stabilities of the as-cast and quenched alloys. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.