Hydrogen sorption kinetics of ball milled MgH2 with and without chemical additives were studied. We observed kinetics and capacity improvement with increasing the number of sorption cycles that contributed to the micro/nano cracking of MgH2 particles, shown by XRD and SEM studies. In addition, to investigate the proposed specific role of O2- -based additives on the sorption kinetics of MgH2, we have undertaken a comparative study evaluating the performances of MgH2 containing the NbCl5, CaF2 or Nb2O5 additives. At 300 degrees C, addition of NbCl5 and CaF2 improved the sorption capacity to 5.2 and 5.6 wt% within 50 min, respectively, in comparison to the required 80 min in the case of Nb2O5. This suggests the importance of the chemical nature of the catalyst for hydrogen sorption in MgH2. In addition, the catalyst specific surface area was shown to be very critical. High surface area Nb2O5 (200 m(2) g(-1)), prepared by novel precipitation method, exhibits an excellent catalytic activity and helped to desorb 4.5 wt% of hydrogen from MgH2 within 80 min at a temperature as low as 200 degrees C. (c) 2006 Elsevier B.V. All rights reserved.