The kinetics of hydriding/dehydriding of magnesium has been significantly improved during the recent years by the introduction of new methods of fabrication of nanocrystalline magnesium-catalyst composites. However, the equilibrium hydrogen pressure for decomposition of nanocrystalline MgH(2) was found to be somewhat lower than for conventional magnesium hydride. Moreover, the essential difference in equilibrium hydrogen pressure for absorption and for desorption of hydrogen by nanocrystalline magnesium was reported by many authors. This difference called "hysteresis" is a common phenomenon for hydrides, but it is not observed for magnesium powders with an ordinary particle size (larger than 1 mu m). The aim of the present work was to elucidate why the hysteresis arises in magnesium-hydrogen system with a decrease in particle size. it is shown that the "hysteresis" observed for nanocrystalline magnesium is an apparent phenomenon which is due to a hindered nucleation during hydriding at a pressure close to equilibrium. The pressures measured for desorption represent the real equilibrium. The plateau pressure of Mg + H(2) <-> MgH(2) equilibrium is lower for nanocrystalline magnesium than that for conventional magnesium. This result is explained in terms of smaller surface energy of magnesium hydride in comparison with magnesium. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.