Extended x-ray absorption fine structure spectroscopy, x-ray diffraction and transmission electron microscopy were used to analyze the Nb coordination and clustering in Nb-doped (5 at. %) h -Mg film samples deposited by rf magnetron sputtering. Results show that the catalytic effect of the Nb doping in the H ₂ absorption and desorption kinetics is connected with the formation of Nb nanoclusters dispersed in the host matrix. The H ₂ desorption from β- MgH ₂ is favored by local elastic stresses produced by β- NbH _0.89 clusters on the MgH ₂ matrix that reduces the stability of the hydride phase and by preferential paths in the nanocomposite hydride.