We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H2 (reactive mechanical grinding) with oxides Cr2O3, Al2O3 and CeO2. The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH2 during reactive mechanical grinding. The Mg+10wt.%Cr2O3 powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO2 and Mg+10wt.%Al2O3. The Mg+10wt.%Cr2O3 powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al2O3 and Mg+10wt.%CeO2. Mg+10wt.%Cr2O3 absorbs 5.87wt.% H at 573 K, 11 bar H2 during 60 min at the first cycle. The Mg+10wt.%Cr2O3 powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO2 and Mg+10wt.%Al2O3. It desorbs 4.44 wt.% H at 573 K, 0.5 bar H2 during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding-dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr2O3, Mg+10wt.%Al2O3 and Mg+10wt.%CeO2. The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr2O3, Mg+10wt.%Al2O3 and Mg+10wt.%CeO2. The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr2O3 in the Mg+10wt.%Cr2O3 powder is reduced after hydriding-dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr2O3 after cycling.< Réduire