Controlled reactive mechanical milling (CRMM) of Mg powder under “ shearing” mode in hydrogen gas for 26.5, 50, 70 and 100 h results in the formation of nanostructured β-MgH 2 hydride. Based on the comparison of integrated X-ray diffraction (XRD) intensities of Mg and β-MgH 2 peaks, it is postulated that during reactive milling up to 50 h the crystalline Mg is mostly consumed to form β-MgH 2. However, from 50 to 100 h the crystalline Mg mostly forms an amorphous phase with only a small fraction of it being consumed for the β-MgH 2 creation. The formation of amorphous Mg was also observed in the 2Mg–Fe mixture subjected to controlled reactive mechanical alloying (CRMA) in hydrogen. A massive formation of Mg(OH) 2 hydroxide is observed by XRD in the powders reactively milled in hydrogen and subsequently exposed to the ambient environment for about 4 months. The formation of Mg(OH) 2 occurs due to hydrolysis of nanostructured β-MgH 2. Abnormally high weight losses on the order of ∼16–24 wt.% are observed during thermogravimetric analysis (TGA) of powders containing Mg(OH) 2 which confirm the release of water from Mg(OH) 2 upon heating. Also, differential scanning calorimetry (DSC) curves show endothermic peaks corresponding to the release of water from Mg(OH) 2 which are in excellent agreement with DSC peaks corresponding to the release of water from Mg(OH) 2 in the 2Mg–Fe 10 h reactively milled and “aged” mixture.