AbstractThe kinetics of hydrogen absorption and desorption (H-sorption) in magnesium have been improved by mechanically milling magnesium hydride (MgH2) with metal oxides (MxOy). Here, we investigate the effect of initial particle size of the oxides on the H-sorption properties of magnesium. As mechanical milling is regarded as an efficient method to break and disperse catalytic particles of MxOy, nanosized MxOy should allow shorter milling times and improved hydrogen kinetics (H-kinetics) compared to microsized MxOy. Our results show that the fastest kinetics can mostly be achieved, independent of initial particle size of the oxides. In some cases, milling time can be shortened to achieve this, while in other cases, nanosized MxOy does not lead to the expected improvement of magnesium H-sorption properties. The results are rationalised in a new frame considering MxOy as process controlling agent helping to refine MgH2 particle size during milling.