To investigate the self-discharge mechanism in Ni/H-2 cells, the interactions of hydrogen with the beta-NiOOH and gamma-NiOOH nickel oxyhydroxide phases were simulated in a water-loaded autoclave as functions of both temperature and pressure, Under such conditions, X-ray diffraction measurements coupled with transmission electron microscopy showed that both gamma(III) and beta(III) phases are reduced to a beta(II) phase, thus providing direct evidence of hydrogen-nickel oxyhydroxide chemical interaction in charged Ni/H2 cells. The kinetics for such a reaction were found to be mainly controlled by temperature rather than hydrogen pressure. Partial dissolution of solid beta-NiOOH and gamma-NiOOH phases, followed by dissolved hydrogen reduction concomitant with an oriented nucleation growth of the reduced phase on the intact NiOOH, is proposed to account for the self-discharge mechanism observed in the autoclave.