The hydrogenation properties of Mg(100-x)Ni(x) alloys (x = 0.5, 1, 2, 5) produced by melt spinning and subsequent high-energy ball milling were studied. The alloys were crystalline and, in addition to Mg matrix, contained finely dispersed particles of Mg(2)Ni and metastable Mg(6)Ni intermetallic phases. The alloys exhibited excellent hydrogenation kinetics at 300 degrees C and reversibly absorbed about 6.5 mass fraction (%) of hydrogen. At the same temperature, the as prepared Mg(99.5)Ni(0.5) and Mg(95)Ni(5) powders dissolved about 0.6 mass fraction (%) of hydrogen at the pressures lower than the hydrogen pressure corresponding to the bulk Mg-MgH(2) two-phase equilibrium, exhibiting an extended apparent solubility of hydrogen in Mg-based matrix. The hydrogen solubility returned to its equilibrium value after prolonged hydrogenation testing at 300 degrees C. We discuss this unusually high solubility of hydrogen in Mg-based matrix in terms of ultrafine dispersion of nanometric MgH(2) precipitates of different size and morphology formed on vacancy clusters and dislocation loops quenched-in during rapid solidification. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.