Nanocrystalline Mg films with thicknesses between 45 and 900 nm were prepared by e-beam on fused-SiO2 substrates and hydrogenated at 280 degrees C to investigate the H-absorption/desorption process. Films were characterized by XRD, RBS, Raman, FEG, "in situ" optical measurements and TPD-MS. Whereas practically full conversion into MgH2 is observed in thinner films (d < 150-200 nm), higher amount of hydrogen is not absorbed by thicker films (d > 200-250 nm) that is attributed to the formation of Mg2Si-MgO phases (observed by RBS and Raman) as well as the slow kinetics of MgH2 formation. H-desorption process is controlled by a nucleation and growth process and hydrogen is released at lower desorption temperatures (T-d = 425 degrees C) than bulk MgH2. T-d are slightly lower (Delta T similar to 25 degrees C) in thickest hydrogenated films (d > 200-250 nm) suggesting an influence of Mg2Si and MgO phases, formed during hydrogenation. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.