A heat conduction calorimeter has been used to determine enthalpies of solid-gas reactions in systems, such as intermetallic compounds-H-2(g). It is shown that the significance of the heat measurements must be carefully analyzed by controlling several parameters, which may influence the heat transfer conditions during the absorption and desorption reactions. For this type of reaction, the measured heat flow is strongly dependent on the heat transfers through the gas phase in the low pressure range (Knudsen regime). It is demonstrated, experimentally and confirmed with simple models, that heat measurements can provide erroneous enthalpies due to our inability to account for the unavoidable modifications of the heat losses in the transition region. Experiments were carried out on the ZrNi-H-2 system.