interaction was detected between the sulfonium centre and solvent molecules having a negatively polarized oxygen atom. The hydrolysis of sulfonium salts yielding sulfoxides was investigated by a kinetic method in 98∶2 (v/v) dioxane–water mixture and in water. On the basis of medium, substituent (ρ +1.03), steric, salt and kinetic isotope effects detailed mechanisms involving a hydroxy-λ4-sulfane intermediate