A kinetic model has been developed, describing the kinetics of the hydrogen storage reactions in hydride-forming materials under equilibrium conditions. Based on first principles chemical reaction kinetics and statistical thermodynamics, the model is able to describe the complex processes occurring in hydrogen storage systems, including phase transitions. A complete set of equations, governing pressure-composition isotherms in both solid-solution and two-phase coexistence regions has been obtained. General expressions for rate constant dependencies have been proposed, using well-defined phase-dependent Hamiltonians for the hydrogen energy state at the surface and in the bulk of hydride-forming materials. The characteristics of both model (LaNiyCu1.0) and commercial, misch-metal-based AB5-type materials at different compositions and temperatures have been simulated. Good agreement between experimental and theoretical results for the pressure-composition isotherms has been found in all cases.