The aim of this work is the study of the H-2 release from the thermal decomposition reaction of sodium tetrahydroboride (NaBH4) in the solid state. Computational and experimental methods have been used. NaBH4 thermodynamic properties at room temperature and its energy of formation from NaH and BH3 have been theoretically studied using a first-principles approach. Results obtained from ab initio calculations compared well with thermodynamic properties assessed by the CALPHAD method. The effect of annealing on phase transformations in NaBH4 has been followed with various experimental analytical techniques and the structure of the samples, after suitable thermal treatments, has been analysed by X-ray diffraction. A multi-step reaction has been observed after annealing, in both isothermal and scanning conditions. The conditions for H-2 release from NaBH4 in the solid state have been clarified combining experimental and computational results. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.