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 degreesC 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 O-2 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 similar to430degreesC with evolution of NO2 and Some O-2. Release from BaO/Al2O3 exhibits an additional peak near 520degreesC corresponding to formation of NO and a higher O-2 concentration. The NO may arise from NO2 since BaO/Al2O3 has activity for NO2 decomposition by 500degreesC. Although CO2 at low concentration is rapidly taken up by BaO/Al2O3 at 400degreesC 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 380degreesC if NO is evolved than if O-2 is consumed.