We examined the deuterium absorption and desorption of 55 nm thick Mg films alloyed with Fe and Cr using in-situ neutron reflectometry. The Mg alloy films were covered with bimetallic catalyst layers and could be fully absorbed at room temperature at a pressure of 8 mbar. The NR experiments revealed a deuterium gradient within the Mg alloy layers during absorption and a large deuterium uptake up to a DIM ratio of about 0.45 before the layer started to expand and form magnesium deuteride (MgD2). Our NR data suggest that the catalytic effect of the Fe-Cr alloy is based on the avoidance of the formation of a blocking MgD2 layer in the early stages of the absorption process leading to a fast hydrogen absorption kinetics. Crown Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.