MgH2 is an important ingredient in modern reactive hydride composites to be used as hydrogen storage materials. The surface composition and chemical state of ball-milled MgH2 is studied during hydrogen desorption by means of X-ray photoelectron spectroscopy. Simultaneously, the desorption rate of hydrogen is monitored, which is compared to dissociative properties of the surface investigated by hydrogen - deuterium exchange experiments. It is found that MgH2 is also oxide covered during desorption demonstrating that MgO is able to recombine atomic hydrogen. The corresponding catalytic sites are associated with low coordinated surface vacancies on the oxide. The maximum surface concentration of these vacancies is very small, which is countered by a very high turnover frequency due to a small activation energy for dissociation of hydrogen of 0.1 eVon the single vacancy. The study provides insight into the catalytic role played by the oxide additives in MgH2, which are superior catalysts for hydrogen sorption even when compared to 3d-metals. (c) 2007 Elsevier B. V. All rights reserved.