The evolution of microstructure during the pressure-composition-temperature (PCT) test at 350 C for Mg-10at%Al and Mg-4at% alloy powders fabricated by electrodeposition as well as pure Mg powder were studied. The results of this study show that the starting microstructure of the Mg-10at%Al alloy powders consist mainly of the hcp-Mg phase. However, due to the rejection of Al by the MgH(2) phase, a considerable amount of the Mg(17)Al(12) phase forms upon the initial hydrogenation stage. Consequently, this powder exhibits three transformational steps with the final microstructure consisting of MgH(2) and fcc-Al phases. The dehydrogenation takes place at relatively high pressure by formation of the Mg(17)Al(12) phase at the MgH(2)/fcc-Al interface. The hydrogen released diffuses through the interconnected fcc-Al/Mg(17)Al(12) network to the surface. In contrast, Mg-4at%Al and pure Mg powders release hydrogen at a low pressure by the nucleation of the hcp-Mg phase on the surface. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.