Nanocrystalline Mg-Ni alloy powder was produced via the magnetron co-sputtering technique. The samples obtained showed crystallisation of Mg and Mg(2)Ni and an overall morphological heterogeneity with different enriched zones, as evidenced by microstructural analysis. A number of pressure-composition-isotherms (PCI) were carried out on these samples at a range of temperatures. Following hydrogenation, different hydrogen sorption behaviours were observed depending on the samples' relative composition in terms of Mg and Mg(2)Ni. Hydrogenation/dehydrogenation cycling is believed to have caused modification of the nanocrystalline materials investigated. The derived dissociation enthalpies of the corresponding hydrides, MgH(2) and Mg(2)NiH(4), were found to be coherent with values reported in literature, with the exception of two samples. In addition, one of these samples showed a third plateau pressure pointing to the presence of a hydrogenated metastable phase. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.