To analyse heat and mass transfer in a metal-hydrogen reactor, the following hypotheses are typically used: (i) solid and gas are at the same temperature, (ii) the effect of pressure variation in the reactor is negligible, (iii) the effect of hydrogen concentration on the equilibrium pressure variation is negligible. In this paper, we test the validity of these hypotheses by comparing numerical results obtained with and without these hypotheses. For a LaNi 5-hydrogen reactor, the results show that these hypotheses are valid under almost all conditions.