Recently, the present authors [17] have reported dramatic improvements in the hydrogenation behaviours of nanostructured LaMg11Ni prepared by Rapid Solidification, caused by modifications of the microstructure and crystal structure. The aim of the present work was to study the mechanism and kinetics of the hydrogen interaction with rapidly solidified LaMg11Ni by employing in situ synchrotron X-Ray diffraction studies of hydrogen absorption-desorption processes in hydrogen gas or in vacuum. These studies uncovered a number of temperature-dependent phase structural transformations contributing to reversible hydrogen absorption and desorption, including (a) formation of metastable (in hydrogen) solid solutions of Ni in La2Mg17 with Ni substitution on both La and Mg sites; (b) amorphisation and nanostructuring of the alloys depending on the solidification rate; nanocrystallisation of the amorphous alloys proceeding at rather low glass transition temperatures and yielding nanocrystallites of Mg2Ni/Mg19La0.1Ni and La1.8Mg17Ni1.0; (c) the mechanism of the Hydrogenation-Disproportionation-Desorption-Recombination processes resulting in a two-step cooperative Mg-assisted phase transformation where a low temperature decomposition of LaH2 led to the recombination of the intermetallics LaMg12 and La2-xMg17Nix. The metastable solid solutions of Ni in the La2Mg17-based intermetallic show high hydrogenation rates and, despite they decompose during the cycling of hydrogen absorption and desorption, the formed on cycling nanocrystallites of Mg2Ni further maintain high catalytic activity of the materials towards the hydrogen absorption. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.