The present work reports on microstructure investigations of hydrogen-loaded nanocrystalline Gd films by means of slow positron implantation spectroscopy combined with in situ synchrotron radiation X-ray diffraction. It is found that the virgin films contain a high density of vacancy-like open volume defects at grain boundaries which trap positrons. These defects represent trapping sites also for hydrogen. With increasing hydrogen concentration the transformation from the alpha-into the beta-phase (GdH2) takes place in the film. Accumulation of hydrogen at grain boundaries causes a decrease of positron localization at defects. The transformation into the beta-phase is completed at x(H) approximate to 1.6 H/Gd. Contrary to bulk Gd specimens, the gamma-phase (GdH3) is not formed in the nanocrystalline Gd films. (c) 2008 Elsevier B.V. All rights reserved.