The chemistry of SO2 on pure metallic Cs and Cs-promoted polycrystalline surfaces of ZnO and MoO2 has been studied using high-resolution synchrotron-based photoemission. Metallic cesium reacts vigorously with SO2 at temperatures between 100 and 300 K. At 100 K, the amount of SO: that fully dissociates (SO2,gas - S-a + 2O(a)) on the alkali metal is relatively small, and SOx (x = 2-4) species predominate on the: surface. SO3 and SO4 may be formed as a result of the reaction of SO2 with atomic oxygen (SO2 + nO(a) - SO2+n,a where n = 1-2), or by disproportionation of chemisorbed SO2. At 300 K the stability of the CsSOx species is lower than at 100 K, and the formation of CsOy and CsSy becomes important. In the Cs/ZnO and Cs/MoO2 systems, the alkali <-> oxide interactions are very strong. Despite this, the supported Cs atoms retain a large chemical affinity for SO2 and are able to enhance the rate of adsorption of the molecule on the oxide surfaces, even at low coverages (0.1-0.2 ML) of cesium when the alkali atoms are in a highly ionic state: (Csdelta+). The larger the coverage of Cs on ZnO and MoO2, the faster the rate of adsorption of SOL, and the larger the amount of S and SOx (x = 2-4) species present on the surface. The adsorption of SO2 on ZnO and Cs/ZnO surfaces was examined using ab initio self-consistent-field (SCF) calculations and cluster models. The Cs adatoms provide occupied states that are very efficient for bonding interactions with the LUMO of SO2. Cs atoms supported on Zn-terminated terraces of ZnO respond much more strongly to the presence of SO2 than Cs atoms supported on O-terminated terraces. The bonding interactions between the ZnO(0001)-Zn face and SO2 are weak, and promotion with Cs adatoms considerably improves the energetics for SO2 adsorption.