Reactive Mechanical Grinding (RMG) under H-2 Of magnesium powder allows to obtain a partial conversion of Mg to MgH2. The ratio MgH2/Mg can be improved by increasing the milling time or by adding a 3d-element M (with the same initial particle size). X-ray diffraction was applied successfully to determine the quantity of MgH2 formed. The RMG allows to suppress the activation procedure generally requested for magnesium. 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. It is demonstrated that RMG for short time (2 hours) is a effective way to strongly improve the hydrogen storage properties of both magnesium and (Mg + 10 wt.%M) mixtures.