Synchrotron-based high-resolution photoemission and first-principles density functional calculations (DFT-GGA) were used to study the interaction of SO2 with clean and modified (OH, Odelta-, O vacancies, or Cu adatoms present) MgO(100) surfaces. The reaction of the molecule with pure and hydroxylated powders of Mf;O was investigated using X-ray absorption near-edge spectroscopy (XANES). At 100 K, the main product of the adsorption of sulfur dioxide on MgO(100) is sulfite (SO2.gas + O-lattice --> SO3,ads) NO evidence is found for bonding of SO2 to Mg sites of the surface or decomposition of the molecule. DFT calculations show that a eta(3)-S,O,O adsorption configuration leads to a SO3-like species, and this is much more stable than configurations which involve bonding to only Mg sites or formation of SO4. On a flat MgO(100) substrate, the formation of SO4 is not energetically viable. A SO3 --> SO4 transformation is observed at temperatures between 150 and 450 K with a substantial reconstruction of the oxide surface. From 450 to 650 K, the adsorbed SO3/SO4 species decompose and SO2 desorbs back into gas phase. The presence of OH groups and 0(delta-) (delta < 2) species on MgO favors the formation of SO4 at the expense of SO3. On the other hand, the creation of O vacancies in MgO(100) by ion sputtering leads to decomposition of SO2. The chemistry of SO2 on Cu/MgO-(100) surfaces is rich. At 150 K, the SO2 molecule chemisorbs intact on the supported Cu particles and forms SO3 on the oxide substrate. Heating to room temperature induces full decomposition of SO2 and the formation of SO4, The Cu adatoms facilitate the decomposition of SO2 by providing electronic states that are very efficient for interactions with the lowest unoccupied molecular orbital (S-O antibonding) of the molecule.