Fast slowing down the hydrolysis Reaction 2 in pure water accompanied by the increase of pH of the reaction solution has its origin in the formation of dense layer of nanocrystalline Mg(OH)2 (weak alkali characterised by very low solubility in water ) which prevents diffusion of water molecules to the surface of MgH2 particles. This is confirmed by our XRD results (Table ) showing presence of Mg(OH)2 nanocrystallites (~11 nm) in the deposit after hydrolysis of MgH2 in deionised water. Conversely, in the deposits after MgH2 hydrolysis in the acidic solutions, Mg(OH)2 was not detected by XRD, and their SEM images (Figure )) were very similar to the one of the starting MgH2 (Figure ). It allows us to conclude that only unreacted starting material (Mg+MgH2) remains after incomplete hydrolysis of MgH2 in the acidic solutions, without formation of detectable amounts of magnesium hydroxide.