A metastable niobium hydride, believed to trigger rapid H desorption from the MgH2 matrix, was detected in the course of real time synchrotron diffraction studies of the desorption kinetics of hydrogen in MgH2 with about 5% NbH nanoparticles. In order to completely characterize this metastable phase, we performed in situ neutron diffraction experiments during hydrogenation/dehydrogenation of a magnesium-niobium nanocomposite. At 250 ࠃ (under 1 bar of H pressure) a hydride phase NbHx appears. Unfortunately, the available low pressure together with the low q-range did not allow to propose a structural arrangement for this phase. We thus switched to standard neutron diffraction (that is at constant pressure and room temperature) of ball-milled Mg-Nb-H nanocomposites for which various NbH hydrides were detected including the solid solution. It turns out that the formation of these niobium hydrides (metastable or not) is closely related to the milling time and depends on the quality of the starting metal.