The degradation mechanism of the La-Mg-Ni-based hydrogen storage electrode alloy La0.7Mg0.3Ni3.4Mn0.1 during charge/ discharge cycling has been systematically studied using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), electrochemical impedance spectroscopy, and linear polarization measurements. The results indicate that the intense pulverization and the oxidation/corrosion of the active components during cycling are the two main factors responsible for the fast capacity degradation, and the degradation mechanism of La0.7Mg0.3Ni3.4Mn0.1 electrode can be classified into the three consequent stages, i.e., the pulverization and Mg oxidation stage, the Mg and La oxidation stage, and the oxidation-passivation stage. SEM observation indicates that the pulverization of the alloy particles is serious due to the high cell volume expansion ratio of the hydride to alloy phase of (La, Mg)Ni-3 and LaNi5. A large number of cracks in both the alloy particles and the electrode are thus produced. XPS and AES analyses reveal that composite passive layers of Mg(OH)(2) and La(OH)(2) are formed continuously on the surface of alloy particles and increase in thickness with cycling. This porous film not only decreases the surface electrocatalytic activity but also retards the transfer of hydrogen into/or from the alloy particles to electrolyte, and decreases the amount of hydrogen absorption elements La and Mg in the alloys as well. © 2005 The Electrochemical Society. All rights reserved.