The H sorption properties of mixtures Mg + WO3 (having various structures) and Mg + H0.23WO3 are reported. First, the higher conversion of Mg into MgH2 during reactive mechanical grinding (under 1.1 MPa of H-2) for higher WO3 content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.