The hydrogen sorption properties of Mg + 10 wt% WO3 and Mg + 5 wt% Cr2O3 mixtures made by reactive (under hydrogen) mechanical grinding were studied and compared with those of elemental Mg subjected to a similar preparation procedure. It was observed that both oxides have an important catalytic effect on hydrogen absorption and desorption. Moreover, in the case of Cr2O3 addition, both milling speed (i.e, milling energy and milling mode) and ball to powder weight ratio influence drastically the hydrogen sorption kinetics. (C) 2004 Kluwer Academic Publishers.