The hydrogenation/dehydrogenation characteristics and hydrogen storage properties of nominal Mg3Ag and Mg3Y alloys prepared by induction melting were investigated. The asmelted Mg3Ag alloy was composed of Mg54Ag17 phase, while Mg3Y consisted of Mg24Y5 and Mg2Y phases. MgmAgi, transformed into MgAg and MgH2 during the first hydrogenation, and the phase transition of the following hy/dehydrogenation cycles was Mg3Ag + 2H(2) <-> MgAg + 2MgH(2). Both Mg24Y5 and Mg2Y undertook disproportion reactions and decomposed into MgH2 and YH3. Experimental and calculated results demonstrated that there was no necessary relation between the thermodynamic stabilities and the size interstices in these alloys. The dehydrogenation enthalpy change (Delta H) and entropy change (Delta S) of Mg3Ag were calculated and compared with that of pure Mg, which indicated that the increase of Delta S could counteract the stabilization effect of Delta H, which offered a method for tuning the thermodynamic properties of Mg-based alloys. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.