The use of cathodic stripping potentiometry for the reliable determination of trace and ultra trace concentrations of selenium in environmental and biological samples on a glassy carbon mercury film electrode is described. The optimum conditions for the method include 3 M HCl as supporting electrolyte, an electrolysis potential of -100 mV vs. SCE, a constant reduction current of -20 μA and the decomposition of the samples by dry ashing with magnesium nitrate as an ashing aid. Under these conditions, the detection limit is 0.8 μg/l with an electrolysis time of 5 min or 0.04 μg/l with 60 min deposition. The relative standard deviation for the measurement at this level is 6% (n = 7). The presence of inorganic and organic substances such as lead, copper, cadmium, zinc, CTMAB, LAS, LPC and Triton X-100 caused some suppression of the selenium peak, but these effects were easily circumvented by the use of standard additions method and a UV-irradiation procedure. The UV-irradiation of the digested environmental and biological materials was also effective in reducing the required dry ashing period to 1 h, as well as in improving the sensitivity and accuracy of the method.