The hydrogen desorption kinetics of composite materials made of magnesium hydride with transition metal additives (TM: Nb, Fe, and Zr) was studied by several experimental techniques showing that (i) a few TM at.% concentrations catalyse the H(2) desorption process, (ii) the H(2) desorption kinetics results stabilized after a few H(2) sorption cycles when TM atoms aggregate by forming nanoclusters; (iii) the catalytic process occurs also at TM concentration as low as 0.06 at.% when TM atoms clustering is negligible, and (iv) mixed Fe and Zr additives produce faster H(2) desorption kinetics than single additive. The improved H(2) desorption kinetics of the composite materials can be explained by assuming that the interfaces between the MgH(2) matrix and the TM nanoclusters act as heterogeneous sites for the nucleation of the Mg phase in the MgH(2) matrix and promote the formation of fast diffusion channels for H migrating atoms.