The mechanism for NO2 storage in barium oxide supported on magnesium oxide (BaO/MgO) was investigated. In situ Raman spectroscopy and XRD were used to follow temporal changes in the composition of the storage material that occurred during the storage process, i.e., during the reaction of 14 mol % BaO with NO2 and NO2/O-2, and the reduction of the stored NO2 with CO. Above 200 degreesC, the interaction of BaO with NO2 leads to both the formation of nitrate and nitrite ions, which, up to 400 degreesC, occurs via an intermediate bariumnitro species. At lower loadings (7 mol %), a destabilization of the barium-nitro species but an enhanced rate for nitrate formation is observed. Similarly, the presence of oxygen enhances the rate for nitrate formation, but suppresses the formation of nitro species. CO reduces the stored NO2 only above 500 degreesC and leads to the formation of thermally stable carbonates. In the presence of NO2, however, they are displaced during the formation of nitrates at 400 degreesC.