The use of a combined working(glassy carbon mercury him electrode)-counter-reference electrode and a rotating sample platform for the reliable determination of selenium in biological and environmental materials by cathodic stripping potentiometry is described. The optimum conditions for the determination of selenium by this method include an electrolyte consisting of 2 M HCl and 1 M CaCl2, an electrolysis potential of -200 mV and a stripping current of -5 μA. Under these conditions, as little as 0.1 μg/l of selenium can be detected with an electrolysis time of 120 s, and the precision at this level was within 1-3% rsd. The interferences of organic and inorganic substances on the cathodic stripping potentiometric determination of selenium were minimal, except for copper which gave an additional peak when present at >2 mg/l. Even in this case, the copper interference can be overcome or reduced by sample dilution. The use of a dry ashing method with the technique enabled the reliable determination of selenium in biological and environmental materials. The direct determination of selenium in water samples by cathodic stripping potentiometry on the combined electrode is demonstrated.