Measurements of equilibrium hydrogen pressure as a function of hydrogen content and of temperature are a convenient way to determine the thermodynamic properties of metal-hydrogen systems. To date such studies have only been carried out for the systems at relatively low hydrogen pressure. We have developed a high-pressure apparatus capable of pressures up to 1.2 GPa and temperatures up to T = 450 K for the studies of equilibrium conditions in the Ni-H systems and the Ni-D systems in order to derive corresponding enthalpies of formation and decomposition. The results show that although the pressures at given temperatures are always higher for (Ni + D-2) than for (Ni + H-2), the values of enthalpies are almost identical within the experimental error. The enthalpies of the formation and decomposition of both systems derived from these studies are compared with calorimetric measurements carried out at high pressure. The difference between enthalpies of formation and decomposition for both systems reflect hysteresis, a common phenomenon in transition metal hydrides.