Reactive Mechanical Alloying (MA in H-2) Of magnesium powder allows to obtain partial conversion of Mg to MgH2. The ratio MgH2/Mg can be improved by increasing the milling time or by adding a 3d-element (with the same initial particle size). X-ray diffraction was applied successfully to determine the quantity of MgH2 formed. The RMA allows to suppress the activation procedure generally requested for magnesium. The hydriding is determined to be a two step process: first nucleation and then diffusion. A direct relationship exists between the nucleation duration and the specific surface. It is demonstrated that RMA for short time (2 h) is an effective way to strongly improve the hydrogen storage properties of both magnesium and (Mg + 10 wt% M) mixtures.