Sulfur dioxide reduction has been studied in dimethylsulfoxide solutions using in situ spectroelectrochemical techniques. Results indicate that the reaction may be described as a CECC (preceding chemical reaction-electron transfer, followed by further chemical reactions) mechanism in concentrated solutions. Sulfur dioxide or its dimer, (SO2)(2), is reduced to a corresponding anion radical, SO2-. or S2O4-., as a first primary electron transfer product depending on the concentration of sulfur dioxide. The radical anion at higher SO2 concentrations then forms an adduct, (SO2)S2O4-., which undergoes a disproportionation reaction to produce the final product, dithionite ion, S2O42-. Minor reaction paths consistent with electrolysis products reported in the literature, such as elemental sulfur (S) and disulfur oxide (S2O), are also discussed.