Using first-principles calculations, we investigated the structural properties and formation energies of H-related intrinsic defects and F- or Cl-related impurities in MgH2. It was found that hydrogen vacancy and interstitial are favorable in charged states, whereas F and Cl impurities tend to substitute for hydrogen atom with the neutral state. We showed that F and Cl cannot lead to decrease of the removal energy for neutral and negatively charged H atom, but both of them can markedly decrease the removal energy for positively charged H atom. This result provides an explanation for the experimentally observed lowering of the onset temperature for hydrogen desorption from TiF3- and TiCl3- doped MgH2. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.