The influence of NO on the adsorption and desorption of NO2 on BaO/TiO2 has been studied under lean conditions. The adsorption of NO involves the disproportionation of NO2 into an adsorbed nitrate species and NO released to the gas phase with a 3:1 ratio,BaO + 3NO(2) --> NO + Ba(NO3)(2).Three different nitrate species form on the catalyst: surface nitrates on the TiO2 support, surface nitrates on BaO, and bulk barium nitrate. The stability of the three species in different gas feeds was investigated by temperature-programmed desorption (TPD).The reverse reaction of the NO2 disproportionation has also been observed. If NO is added to the feed, nitrates previously formed on the sorbent will decompose into NO2. Therefore, the above chemical equation should be considered as an equilibrium reaction. Applying this finding to the NO, storage and reduction catalyst means that NO probably reacts with the previously formed nitrates yielding NO2 as an intermediate product. This NO2 is subsequently reduced by the reducing agents (hydrocarbons and CO) present during the regeneration period. (C) 2003 Elsevier Science B.V. All rights reserved.