The effects of Mn content on the structure and the main electrochemical properties of La 0.7 Mg 0.3 Ni 4.25-x Co 0.75 Mn x (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys were investigated systematically. X-ray powder diffraction (XRD) analysis shows that all the alloys mainly consist of a (La,Mg)Ni 3 phase and a LaNi 5 phase. The lattice parameters and the unit cell volumes of the two phases increase with increasing x. P-C-T curves reveal that the plateau pressure decreases progressively and the hydrogen storage capacity increases first and then decreases with an increase in Mn content. Electrochemical studies showed that the maximum discharge capacity and the high rate dischargeability (HRD) of the alloy electrodes increased with increasing x. Electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization and potential-step measurements, showed that the exchange current density I 0 , the limiting current density I L and the hydrogen diffusion coefficient D all first increase and then decrease with increasing x from 0 to 0.5. Considering the global effect of Mn substitution for Ni on the overall performance of the La 0.7 Mg 0.3 Ni 4.25-x Co 0.75 Mn x alloy electrodes, the optimum composition was found to be x = 0.4.