To characterize UO2 for its possible use in desulfurization applications, the interactions of molecular sulfur dioxide (SO2) with a polycrystalline uranium dioxide film have been studied by means of X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy ion scattering (LEIS). The stoichiometric, oxygen-deficient, calcium-precovered and sodium-precovered UO2 surfaces have been characterized. The changes in oxide reactivity upon creation of oxygen vacancies and coadsorption of sodium and calcium have been studied. After creation of a reduced UO2-x, surface (x similar to 0.44) via Ar+ sputtering, the U 4f XPS spectrum shows conspicuous differences that are good indicators of the surface stoichiometry. Molecular SOx formation (x = 2-4) is observed after SO2 deposition onto stoichiometric UO2 and onto UO2 precovered with small amounts (< 1 ML) of Na or Ca; complete dissociation Of SO2 is not observed. Heating leads to desorption of the SOx species and to transformation Of SO2 to SO3 and SO3 to SO4. On oxygen-deficient UO2 and on UO2 precovered with large Na or Ca coverages (>= 4 ML), both the formation of SOx species and complete dissociation Of SO2 are observed. A higher thermal stability of the sulfur components is observed on these surfaces. In all cases for which dissociation occurs, the XPS peak of atomic sulfur shows similar structure: three different binding states are observed. The reactivity of oxygen-deficient UO2 and sodium-and calcium-precovered UO2 (coverages >= 4 ML) is attributed to charge transfer into the antibonding LUMO of the adsorbed molecule.