Structure changes during hydrogenation are observed in pure Mg, Mg(2)Ni intermetallic (I) and Mg eutectic alloy - 23.5 wt.% Ni (E). Samples were prepared by (i) ball-milling and compacting (alloys I and E) and (ii) by mould casting (Mg and alloy E). Phase composition was checked by SEM and XRD. It was found that the hydrogenated cast alloy I and ball-milled alloy I hydrogenated below the transition temperature T(tr) = 508 K contained a much higher amount of low-temperature un-twinned phase LT1 than the ball-milled alloy I hydrogenated above T(tr). It was shown that micro-twinned phase LT2 slows down the rate of hydrogen desorption. Persistent changes of morphology were observed in all materials after the first hydrogen charging cycle which may explain the so-called activation of Mg-based hydrogen-storage materials described in the literature. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.