A procedure is developed for the photometric determination of selenium in aqueous media using 2-(p-nitrophenyl)-3,5-diphenyltetrazolium chloride. To selectively determine selenium, a two-stage scheme of sample preparation is chosen. It includes the gas extraction of selenium liberated as hydrogen selenide, its extraction from the gas phase to a solution of lead nitrate by liquid absorption and repeated extraction of hydrogen selenide, followed by its absorption with an aqueous reagent solution to form water-insoluble formazan. The formazan is extracted with chloroform. The concentration of selenium is determined from the absorbance of the chloroform extract of formazan. The procedure involves no special preconcentration operations and allows from 10 to 100 μg/L selenium to be determined.