The statistical theory of hydrogen-sorption properties of compounds of alkaline and alkaline-earth metals M(BC)(n)H-x (M = Li, Mg; 0 <= x <= 12, n = 1,2) has been developed in this paper in the expectation that such hydrogenated boron carbides will be the reliable materials for the reversible accumulation and storage of hydrogen in large quantities in perspective. The calculation of free energy of these crystals has been performed on the basis of molecular-kinetic notions, the equation of thermodynamic equilibrium of such system, determining the P-T-c phase diagram, has been derived in the present paper. The hydrogen solubility in these compounds has been ascertained in dependence on temperature and external pressure, the possibility of manifestation of hysteresis effect has been justified. The derived formulae allow to establish the P, T-conditions of high hydrogen content in boron carbide systems and can permit to select the optimum composition of material choosing for hydrogen storage, the regime of technological process, to develop the experimental technology for solving of the practical problems, if in this case the energetic parameters of these materials are known from independent experiments. Copyright (c) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.