Due to increasing energy demands, interest in hydrogen storage is substantial. Magnesium is one of the most attractive systems, yet, development for practical application remains challenging. By combination of X-ray diffraction, electron microscopy and in-situ measurements of resistivity we determine the diffusion coefficient of hydrogen in MgH2 at technically relevant pressure (20 bar). Pd coated thin films of well-defined thickness enable a quantitative evaluation of the hydrogenation rate. From this, we detect linear to parabolic kinetic transition and obtain the diffusion coefficient of hydrogen in MgH2. Measurements at different temperatures (RT-300 degrees C) demonstrate an Arrhenius behaviour with an activation energy E-a = 28.1 kJ mol(-1). This low value and the transformation into a nano-crystalline microstructure upon hydrogenation indicate grain boundary diffusion as the essential mechanism. In completion, the interface Pd/Mg is studied. Mg5Pd2 and Mg6Pd form at elevated temperatures required for dehydrogenation. These phases affect, but do not prevent, further hydrogen loading. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.