The present work is devoted to monitor the reactivity with hydrogen of various oxide systems (Nb(2)O(5), WO(3) and a mixed Mg/Nb/O oxide) in the aim of understanding the role of oxides as promoters for hydrogen storage in MgH(2). The reactivity of the oxides has been tested using either molecular hydrogen or "nascent" hydrogen produced by reaction of zinc with hydrochloric acid. Thermal desorption-mass spectrometry experiments indicate that hydrogen adsorbed in Nb(2)O(5) and Mg/Nb/O mixed oxides is, in part, reversibly released and, for a second fraction, released as water. By contrast. water is the only product desorbed by WO(3) after contact with hydrogen. This could explain the higher performances of Nb(2)O(5) as kinetic promoter of hydrogen storage in comparison with WO(3). (C) 2008 Elsevier Ltd. All rights reserved.