The quantity of accommodated and distribution profiles of hydrogen in 1.5-mu m thick co-sputtered MgNi films after uptake of hydrogen at 800 kPa pressure within the temperature range 200-250 degrees C during 1 h, 3 h, 6 h and 72 h are measured. The occurring phase changes are followed by X-ray diffraction measurements at room temperature. We conclude that the hydrogenation process involves two stages: (i) the fast nucleation of the initial Mg2NiH4 layer near the substrate interface and (ii) the slow random nucleation of the same phase within the remaining part of the film. The growth of the initial hydride layer may be blocked by the surface oxide barrier layer formed during hydrogenation. We find that hydrogen-induced structural transformations are correlated with oxygen contamination and modify hydrogen storage properties. (C) 2009 Elsevier B.V. All rights reserved.