CO2 incorporation in solids is attracting considerable interest in a range of energy-related areas. Materials degradation through CO2 incorporation is also a critical problem with some fuel cell materials, particularly for proton conducting ceramic fuel cells. Despite this importance, the fundamental understanding of the mechanism of CO2 incorporation is lacking. Furthermore, the growing use of lower temperature sol gel routes for the design and synthesis of new functional materials may be unwittingly introducing significant residual carbonate and hydroxyl ions into the material, and so studies such as the one reported here investigating the incorporation of carbonate and hydroxyl ions are important, to help explain how this may affect the structure and properties. This study on Ba2TiO4 suggests highly unfavorable intrinsic defect formation energies but comparatively low H2O and CO2 incorporation energies, in accord with experimental findings. Carbonate defects are likely to form in both pristine and undoped Ba2TiO4 systems, whereas those based on H2O will only form in systems containing other supporting defects, such as oxygen interstitials or vacancies. However, both hydroxyl and carbonate defects will trap oxide ion defects induced through doping, and the results from both experimental and modeling studies suggest that it is primarily the presence of carbonate that is responsible for stabilizing the high temperature α′-phase at lower temperatures.