When magnesium powder containing small additions of certain multiple valence transition metal oxides (TMOs) is ball milled in hydrogen, the hydrogenated Mg-based product can show remarkable improvements in hydrogen absorption/desorption properties. Using a magnetically controlled Uni-Ball-Mill, small amounts of the iron oxides, Fe 2O 3 and Fe 3O 4, were ball milled with Mg powder in a hydrogen atmosphere (Mg to Fe atomic ratio; 20:1). Milling products as well as samples used in hydrogen absorption/desorption experiments were characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. TG analysis combined with DSC revealed a higher hydrogen storage capacity for the Mg + Fe 2O 3 + H 2-milled product (6 wt% H) compared with 5 wt% H for Mg + Fe 3O 4 + H 2. XRD revealed that during heating, both iron oxides were reduced to pure Fe, a result not previously reported for similar materials milled using different milling devices. For both samples, there was little difference found in the decomposition temperature of the as-prepared MgH 2 and rehydrogenated composites. However, storage capacity degradations were observed for the rehydrogenated composites (4 wt% H storage capacity for MgH 2 + Fe 2O 3 and 4.4 wt% H for MgH 2 + Fe 3O 4). The higher capacity degradation of rehydrogenated MgH 2 + Fe 2O 3 composite is also believed to be a result of the reduction reaction, during which more magnesium was consumed than was consumed by the same amount of Fe 3O 4. The results also were related to the particular ball-milling equipment and low-energy shearing milling mode employed, which promoted the development of a nanostructural hydride product which subsequently changed structure significantly during the first desorption cycle.