The adsorption and reaction of NO2 on a thick (>30 ML), pure BaO film deposited onto an Al2O3/NiAl(110) substrate were investigated using temperature-programmed desorption, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy techniques. For the first time, it was clearly demonstrated that BaO reacts with NO2 to initially form nitrite-nitrate ion pairs by the cooperative adsorption mechanism predicted by theoretical calculation. These NO2-/NO3- ion pairs readily form even at 90 K. In the decomposition process of these ion pairs, first the nitrite species release an NO molecule and then form BaO2. At higher temperatures, nitrate species decompose in two steps: at lower temperature as NO2 only, then, at higher temperature, as NO + O-2. The results of NO2 adsorption/reaction on this model system are identical to those we have found on a high surface area 20 wt % BaO/gamma-Al2O3 sample with the exception of surface nitrates that were only observed on the high surface area material.