We have carried out a systematic study of the kinetics of decomposition of stoichiometric and nonstoichiometric magnesium hydride. Using the thermal-desorption methods in barometric modification and mathematical modeling, we have shown that hydrogen desorption from stoichiometric MgH₂ proceeds in two stages: the formation of nuclei of the metal phase and hydrogen liberation through surface islands of the metal phase under the limiting influence of desorption rate. In the course of hydrogen liberation from partially hydrogenated magnesium, the first stage is usually absent. We have established that the influence of other reactions on the overall degasification rate is at least much smaller than the effect of desorption from the surface of α-Mg. We have obtained estimates of the parameters affecting the desorption kinetics. Finally, we have shown that, for analysis of the kinetics of hydrogen liberation from MgH₂, the choice of physically substantiated models has serious advantages as compared with the Avrami-Erofeev approach.