Density functional theory (DFT) quantum chemical calculations are used to determine adsorption energies and geometries of NO, NO2, CO2, and H2O on a barium oxide (100) surface. The study includes two adsorption geometries for NO2. All species form thermodynamically stable adsorbates, and adsorption strength increases in the order NO2 < H2O < NO <= CO2. The influence of surface coverage on adsorption energy is investigated for all species, and a strong coverage dependence is observed. For CO2, a chemisorbed, carbonate-type structure is identified; the adsorption from the gas phase is nonactivated. Numerical calculations of the competitive adsorption/desorption equilibria of the four species show that, under typical engine exhaust gas composition, the BaO surface is carbonated to a large extent. The results indicate that carbon dioxide plays an essential role in the surface processes during NOx storage on BaO, where it can block a large part of available surface sites.