By 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.