The kinetics of hydrogen desorption from storage materials in principle depend on the crystalline phase of the material. In MgH2, desorption rates may be higher in the crystalline gamma phase compared to the equilibrium bulk alpha phase. It has been suggested [R. A. Varin, T. Czujko, Z. Wronski, Nanotechnol., 17 (15) (2006) 3856-3865] that this effect is responsible for enhanced desorption from ball-milled MgH2, since smaller particles contain a higher proportion of the metastable gamma phase. We investigate hydrogen transport kinetics in these phases of MgH2 by using first-principles calculations based on density functional theory. Imposing charge neutrality, we find that the formation energy of hydrogen vacancies in gamma-MgH2 is smaller by 0.032 eV compared to alpha-MgH2. Our calculations of migration barriers show that the only relevant point defect for mass transport in both crystal structures is the positively charged hydrogen vacancy, and that its lowest migration barrier in gamma-MgH2 is 0.02 eV lower than in alpha-MgH2. We conclude that hydrogen vacancies exist in higher concentrations and are also more mobile in the, gamma phase than in the alpha phase, thus explaining the faster dehydrogenation kinetics of gamma-MgH2. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.