We review our understanding of the NO2 interaction with BaO. The presented picture has evolved from calculations using the density functional theory, reactor measurements and vibrational spectroscopy of surface species during NO2 storage on a BaO/Al2O3 catalyst and BaO powder. The DFT calculations predict nitrite formation on the BaO(1 0 0) surface upon NO2 adsorption. A particular stable adsorption configuration is a nitrate-nitrite pair geometry, with the nitrate bonded to a barium cation and the nitrite involving a surface oxygen anion. This configuration is supported by vibrational spectroscopy. Only nitrite formation is observed for NO exposure to BaO powder at low temperatures, whereas signatures at similar to250degreesC reveal the formation of surface nitrates. Continuous NO2 adsorption at this temperature results in surface Ba(NO3)(2), as well as bulk Ba(NO3)(2), formation. (C) 2004 Elsevier B.V. All rights reserved.