Hydrogen diffusion behavior in Mg2Ni-type hydrogen storage alloy electrodes was characterized by using both potentiostatic polarization method, based on a spherical diffusion model, and electrochemical impedance spectroscopy (EIS) technique. The values of the diffusion coefficients of hydrogen in Mg2Ni and Mg(1.)9Vo(0.1)Ni(0.)8Al(0.2) are 1.2 x 10(-11) similar to 1.57 x 10(-10) and 3.1 similar to 7.6 x 10(-9) cm(2)/s, respectively. The values of diffusion coefficients decline notably with the increase in depth of discharge (DOD). The decay rate of diffusion coefficient in Mg2Ni with DOD is much quicker than that in Mg1.9V0.1Ni0.8Al0.2. The experimental results suggested that the substitutions of V and Al for Mg and Ni in Mg2Ni could significantly improve the hydrogen diffusion performance in this alloy and thus substantially increase discharge capacity of the electrode.