The use of reactive mechanical grinding (MG under H2) of magnesium powder improves the hydrogen sorption properties. The hydrogenation of Mg starts in situ during the milling process, thus circumventing the activation procedure that is generally required for Mg. The effects of the addition of various elements or compounds have been studied. The hydriding is determined to be a two-step process: nucleation and diffusion. A direct relationship exists between the nucleation duration and the specific surface area of the magnesium powder. A critical milling time exists up to which the diffusion process is improved and above which no more improvement is observed (the maximum internal stress in the powder is also reached at this critical time). The diffusion is controlled by the number of crystallites per particle that can be reduced by increasing the milling time up to 10 hr. The addition of Co (catalyst), YNi (hydrogen pump), or oxides (abrasive elements and nucleation centers) leads to an improvement of the hydrogen sorption properties (but a strong dependence upon the milling time is reported). Finally, the sorption properties of these mixtures are comparable with those reported for MgH2-metal mixtures.Read less <