The storage and release of NO₂ on alumina-supported barium oxide has been studied with particular attention to the stoichiometry of the two processes. At 400 °C the storage process is characterised by a short period of complete uptake, possibly as nitrito or nitro species, followed by a slower partial uptake in which approximately one NO is released for every three NO₂ lost. The latter reaction appears to supply the oxygen necessary to store NO₂ as nitrate ions. Molecular O₂ has little direct involvement even if in large excess. The second storage reaction also occurs, but to a much lesser extent, with Al₂O₃ alone. During temperature programmed desorption, release of NOₓ from Al₂O₃ peaks at ~430 °C with evolution of NO₂ and some O₂. Release from BaO/Al₂O₃ exhibits an additional peak near 520 °C corresponding to formation of NO and a higher O₂ concentration. The NO may arise from NO₂ since BaO/Al₂O₃ has activity for NO₂ decomposition by 500 °C. Although CO₂ at low concentration is rapidly taken up by BaO/Al₂O₃ at 400 °C it is displaced by NO₂ and does not interfere with storage. Thermodynamic calculations show that the formation of Ba(NO₃)₂ by the reaction of NO₂ with bulk BaCO₃ under the conditions used here is more favourable above 380 °C if NO is evolved than if O₂ is consumed.