A new microelectrode technique was developed for studying hydrogen diffusion in Mg2Ni alloy electrodes during charging and discharging in 6 M KOH solution based on a spherical core shrink model. The diffusion coefficients of hydrogen in α- and β-phase hydrides were measured on a single particle electrode using the potential-step method. It was found that the diffusion of hydrogen in the β-phase of the hydride occurs more slowly than in the α-phase. This effect is attributed to formation of a β-phase hydride layer beneath the particle surface during charging which retards hydrogen transport and results in incomplete charging. This leads to an extremely low electrochemical capacity. The diffusivity of hydrogen in magnesium alloy powders measured by the microelectrode technique was also compared with those evaluated by a porous plate electrode.