Powders mixtures of nanosized MgH2 and suitable additives, obtained by high energy milling, have been studied as materials to be used in a functional solid state hydrogen reservoir. A prototype of a two stages reservoir is under development (patent pending). The hydrogen release from the main stage, with high capacity Mg-based hydrides, is primed by a primer stage containing commercial hydrides able to operate at room temperature. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.