The effect of microstructure on the phase composition and hydrogen absorption-desorption behaviour of Mg-based Mg-20Ni-8Mm (wt.%) (Mm = La-rich Mischmetal) alloys has been studied. Rapid solidification (RS) processing resulted in the formation of the high-temperature cubic modification of Mg(2)Nia(4) and the solid solution hydride Mg2NiH0.3, in the disappearance of the monoclinic modification of Mg2NiH4, as well as in a decrease in the unit cell volumes of the constituent hydride phases. The above-mentioned tendencies became more pronounced in the order "as-cast < Cu-300 < Cu-1000 Cu-2000" (where the sample names Cu-#### denote the spinning velocity of the copper wheel in rpm), which is explained by an increase in the mechanical stresses in the materials and/or by an increased interfacial energy of the fine grains of the corresponding hydrides. The hydrogen absorption kinetics was improved in the order "Cu-300 < Cu-1000 < Cu-2000". The temperature range of hydrogen thermal desorption from the hydrogenated alloys shrank in the order "Cu-300 > Cu-1000 Cu-2000", which is explained by increased uniformity of the hydrides grain size in the hydrides with increasing solidification rate. During PCT (pressure composition temperature) tests, the Cu-1000 and Cu-2000 samples displayed the largest pressure hysteresis and the smallest slope of the higher Mg2NiH4 plateau, but also the lowest hydrogen storage capacity. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.