Nanocrystalline Mg films with thicknesses between 45 and 900 nm were prepared by e-beam on fused-SiO2 substrates and hydrogenated at 280 °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 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 (Td = 425 °C) than bulk MgH2. T d are slightly lower (ΔT ∼ 25 °C) in thickest hydrogenated films (d > 200-250 nm) suggesting an influence of Mg 2Si and MgO phases, formed during hydrogenation. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. ; Authors also thank Spanish MINECO and CAM by financial support under contracts MAT2011-22780, MAT2011-27911 and CCG10-UAM/ENE-5245, respectively. ; Peer Reviewed