The concentration of Se(IV) in zinc plant electrolyte is determined by cathodic stripping voltammetry (CSV) in ammonium sulfate electrolyte containing EDTA and Cu. The Cu concentration has to be adapted to the sample and the deposition time. With voltammetry only free selenium is determined, therefore it has to be taken into consideration that selenium forms sparingly soluble compounds with numerous cations (e.g. Fe2(SeO3 )3 with Ks = 2·10-31).