The adsorption and oxidation of SO2 on alumina and sodium-impregnated alumina has been examined using thermogravimetric analysis and diffuse reflectance infrared Fourier transform spectroscopy. Sulfur dioxide chemisorbs initially at basic sites to form an adsorbed sulfite, which is quantitatively converted to sulfate on oxidation. It has been observed that at low coverages, similar to 2.6 mu mol/m(2), sodium acts as a promoter for the formation of an adsorbed sulfite and sulfate which have structures similar to those of aluminum sulfite and sulfate, respectively. At higher sodium loadings, a second type of adsorbed SO2 is formed, similar to sodium sulfite and sulfate. The species with the aluminum sulfate structure appears to be more easily decomposed than does the sodium sulfate species and accounts for the regenerable adsorption capacity. Formation of the sodium sulfate species appears to account for the loss of adsorption capacity as the number of adsorption/regeneration cycles increases. Oxidation of the sulfite form to the sulfate form can occur in the absence of added oxygen, but it is an activated process and begins to occur in measurable amounts at temperatures between 150 and 300 degrees C. Partitioning of adsorbed SO2 between aluminum and sodium forms is not a function of temperature and depends on only sodium loading.