In this paper, the effect of hydrogen concentration on the reaction enthalpies of some metal hydride alloys during hydriding and dehydring is presented. Pressure-concentration-temperature characteristics of the metal hydride alloys are measured under nearly isothermal condition during both absorption and desorption. Reaction enthalpies and entropies of LaNi(5), LaNi(4.7)Al(0.3), LmNi(4.91)Sn(0.15), Ti(0.99)Zr(0.01)V(0.43)Fe(0.09)Cr(0.05)Mn(1.5) and MmCo(0.72)Al(0.87-)Fe(0.04)Ni(3.91) are estimated by constructing van't Hoff plots at different hydrogen concentrations. it is observed that the effect of hydrogen concentration on reaction enthalpies is more significant for the alloys having larger plateau slopes. At the initial stage of hydrogenation, metal hydrides are found to have larger reaction enthalpies which decrease gradually by about 5-15% at the end of the hydrogen absorption. At any given temperature, desorption enthalpies of LaNi(5), LmNi(4.91)Sn(0.15), MmCo(0.72)Al(0.87)Fe(0.04)Ni(3.91), LaNi(4.7)Al(0.3) and Ti(0.99)Zr(0.01)V(0.43)Fe(0.09)Cr(0.05)Mn(1.5) are found to be higher by about 5, 8, 10, 28 and 32% than their respective absorption enthalpies. Reaction enthalpies of the selected metal hydride alloys are expressed as a function of hydrogen concentration by a fourth order polynomial equation obtained from fitting with the experimental data. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.