Charged NO, species (x = 1,2,3) formed upon adsorption of NO2 on BaO are characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and theoretical vibrational analysis using the density functional theory (DFT). Experiments at 30degreesC reveal nitrite formation. At T > 250degreesC, shifts associated with formation of nitrates are observed, indicating an interconversion of oxygen atoms among adsorbates. The theoretical study includes single and pairwise NO2 adsorption on (BaO)(9) clusters. As has been reported previously [Broqvist, P.; Panas, I.; Fridell, E.; Persson, H. J. Phys. Chem. B 2002, 106, 137], an additional energy gain is calculated for the second adsorbed NO2. A vibrational analysis of the investigated adsorption configurations supports the interpretation of nitrite to nitrate interconversion on the BaO surface. Moreover, the calculations demonstrate the sensitivity in the NO2 vibrational band splitting with respect to adsorption configuration.