Electro-oxidation of sulfite to dithionate, which is catalyzed by copper ion, has been studied by potential step chronoamperometry and potentiometry. The following mechanism has been proposed : Cu-I(SO3)(3)(5-) is oxidized to Cu-II(SO3)(3)(4-), and the latter dimerizes to Cu-2(II)(SO3)(6)(8-), which then decomposes to S2O62- and Cu-I(SO3)(2)(3-) in the rate-determining step. Formation constants of sulfito-copper(I) and -copper(II) complexes have been reported.