Hydrogen generation by hydrolysis of Mg and MgH2 has been investigated in pure water and 1M KCl It has been found that hydrolysis reaction of Mg and Mg-Ni composite, both obtained by high-energy ball milling, is faster and extensive when they are immersed in 1 M KCl. In contrast, milled Mg and Mg-Ni composite in pure water, MgH2 and MgH2-Ni composites in pure water and in 1M KCl show low yield and reactivity. Hydrolysis kinetics and yield are maximum with Mg-10 at% Ni composite milled for 30 min, so reaction is fully completed within an hour in the presence of chloride ions. It is related to the creation of micro-galvanic cells between Mg and dispersed Ni elements, accentuating greatly Mg corrosion in highly conductive aqueous media. A significant increase of the H-2 production is also observed with 30 min milled Mg sample, likely because of the accentuation in the pitting corrosion resulting from the creation of numerous defects and fresh surfaces through the milling process. On the other hand, intensive ball milling of pure magnesium has no effect on the Mg reactivity in pure water. Ball milling effect is likely masked by the significant Mg passivation in pure water. A correlation is established between the conversion yield of ball-milled MgH2 powder in pure water and its effective surface area, which is increased by the milling process. Ni addition has no effect on the hydrolysis reaction in nonconductive media (i.e. pure water) and with nonconductive material (i.e. MgH2). (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.