The Ni/H-2 self-discharge kinetics were studied by means of Ni/H-2 cells equipped with a pressure sensor to determine the time-dependence of cell pressure. The cell pressure variation was found to depend strongly upon cell state of charge (e.g., whether the nickel oxyhydroxide was of beta- or gamma-type). For overcharged Ni/H-2 cells (e.g., having a mixture of beta-and gamma-NiOOH as the nickel electrode) two different rate processes were observed for the self-discharge rate as a function of time, For longer times the curve could be well fitted by assuming a single self-discharge mechanism corresponding to the direct reduction of beta(III) by hydrogen. In contrast, for shorter times the first part of the P(H-2) = f(t) experimental curve exhibited a higher tangential slope, suggesting most likely a combination of two self-discharge mechanisms, (i) the direct reduction of gamma-phase by hydrogen and (ii) electrochemical reduction involving the catalytic oxidation of hydrogen at the sintered matrix.