A new approach to reduce the cycling capacity degradation of Mg-Ni-based alloy through lowering the polarization resistance by means of element substitution is suggested in this study. A new Mg-based amorphous alloy Mg45Fe5Ni50 was prepared by means of mechanical alloying. The cycling stability of this alloy which is found superior to that of Mg50Ni50 and several other A side-substituted Mg-based ternary alloys. A comparative study on the surface layer formed on Mg50Ni50 and Mg45Fe5Ni50 during cycles was made by the X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). XPS analysis demonstrates that, the degree of oxidation of the two alloys are almost the same. Mg on the surface of both alloys transforms into Mg(OH)(2) passive film and the Mg (2P) binding energy peaks of both alloys before and after different cycles are also similar. The only cause for the difference in cycling stability of the two alloys is the difference in polarization resistance R-P, which leads to different potential drops in the charge/discharge processes. On the basis of the EIS Nyquist diagrams and corresponding equivalent, the R-P values of different cycles of both alloys were calculated through non-linear least-squares fitting. For Mg45Fe5Ni50, the polarization resistance is lower. The metallic Fe was believed to be beneficial for suppressing the polarization resistance R-P and elevating the cycling discharge plateau, thus improving the cycling discharge-ability. This phenomenon may be useful in preparing more durable Mg-Ni-based electrode alloys. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.