In solutions of hydrated SO2 it is well-known that both the bisulfite ion HOSO2- and the sulfonate ion HSO3- are present whereas the sulfonate form prevails in some salts. Here we show by ab initio and transition state theory considerations how the mechanism of sulfonate formation works. In aqueous solution, dissolved SO2 first forms a bisulfite ion which is in a next step converted into the sulfonate ion. Direct formation of the sulfonate ion is kinetically hindered due to a very high reaction barrier for this process. Tautomerization of the bisulfite ion into the sulfonate ion is catalyzed by water molecules. Insight into this process is vital in understanding the formation of sulfonate salts upon crystallization from aqueous solution.