Magnesium hydride is considered to be one of the most interesting alternatives for the reversible storage of hydrogen. It is abundant, inexpensive, easy to handle, environmentally benign and exhibits a high hydrogen storage capacity of up to 7.6 wt.%. Furthermore, nanocrystalline Mg powder prepared by high energy ball milling and the addition of suitable catalysts shows very fast absorption and desorption kinetics. The thermal stability of the nanocrystalline microstructure as well as the respective sorption kinetics of ball-milled MgH 2 with or without 0.5 mol% Nb 2O 5 as catalyst have been investigated after cycling and annealing at the technically relevant temperatures between 300 and 400  ° C. While kinetics for pure MgH 2 slows down substantially already after a few cycles at 300  ° C, MgH 2 with Nb 2O 5 catalyst still shows fast sorption kinetics after annealing up to 370  ° C. At higher temperatures, the kinetics for the catalyzed material also breaks down, which is attributed to a deterioration of the catalyst. Continuous coarsening of the microstructure during annealing leads to an increased fraction of the storage capacity that can only be recharged at a slower rate. This is discussed in terms of retarded growth conditions for the MgH 2 phase.