A sensitive analytical technique for the determination of soluble selenium species [Se(IV), Se(VI), Se(0,-II), dimethyl selenide (DMSe), and oxidized methylated organo-Se compounds], using hydride generation atomic absorption spectrophotometry is described. Dimethyl selenide is purged out of a solution and trapped on a U-tube immersed in liquid N2. After controlled heating of the sample trap, DMSe is quantified in a quartz flame-in-tube atomizer aligned in the optical path of an atomic absorption spectrophotometer. Selenite [Se(IV)] and oxidized methylated Se-compounds are reduced by NaBH4in 4 M HCl to H2Se and DMSe, repectively. Entrained by a He carrier gas the H2Se and DMSe are trapped in a liquid N2cooled U-tube. Their separation is accomplished by controlled heating of the sample trap.