In BaTiO 3 x H y two possible mechanisms for hydride ion diffusion have been suggested. The first can be described as an oxidative diffusion, where the hydride ion transforms into a proton followed by interstitial diffusion. In the second mechanism hydride ions move through the material with the assistance of oxygen vacancies. Several investigations using complementary methods, such as isotope exchange, quantum chemical calculations , quasielastic neutron scattering (QENS) and inelastic neutron scattering (INS) have indicated that the latter mechanism most likely occurs in BaTiO 3 x H y oxyhydride and other ATiO 3 x H y pervoskites. While the majority of the studies are in agreement on the hydride ion transport mechanism in these solids, controversies still remain on the electronic structure and the associated electrical conductivity.