The coadsorption of Na and SO 2 on MgO(100) was studied using synchrotron based high-resolution photoemission, X-ray absorption near-edge spectroscopy and first-principles density functional calculations. The adsorption of SO 2 on a MgO(100) single crystal and MgO(100) films, grown epitaxially on a Mo(100) substrate, produces SO 3 and SO 4 species at 300 K. In general, the epitaxial films are a good model for studying the chemistry of SO 2 on MgO(100). Defects and imperfections in the MgO(100) films facilitate SO 4 formation, but no dissociation of SO 2 is observed on the films or MgO(100) single crystal. Dissociation of S-O bonds is seen after coadsorbing SO 2 with Na on the MgO(100) surfaces. The Na adatoms provide occupied electronic states above the MgO valence band which are very efficient for bonding interactions with SO 2 . In the {Na+SO 2 }/MgO(100) system, Na transfers electrons into the lowest-unoccupied molecular orbital of SO 2 inducing a weakening of the S-O bonds that facilitates dissociation of the molecule.