AbstractBy use of a capillary electrophoretic method for anion analysis the anodic oxidation of thiosulfate, present in color photo bleach‐fix solutions, was investigated. In a specially constructed electrolysis cell with a cation‐exchange membrane separating the electrode chambers and with a carbon flow‐through three‐dimensional anode the two‐stage anodic oxidation of S2O32− anions was realized. In the first stage, S2O32− anions were oxidized to a maximal degree to an intermediate compound – tetrathionate (S4O62−) – then S4O62− anions were oxidized to the sulfate (SO42−), avoiding the decomposition of intermediate compounds and the formation of harmful sulfur compounds.© 2002 Society of Chemical Industry