Elemental Se, produced by the reduction of Se^IV with 2-mercaptoethanol can form a zerovalent Se complex with the formula SeL₆ with an excess of the ligand in basic medium. The complex is soluble in water and shows maximum absorption at 380 nm, and this was used as the basis for the spectrophotometric determination of Se. Beer's law is obeyed over the concentration range 2.4–487 p.p.m. of Se. The molar absorptivity of the complex is 323 l mol^–1 cm^–1 and the working pH is between 10 and 13. As regards interfering ions, hydrolysable metal cations such as Fe^III and Cu^II can be precipitated as their hydroxides prior to the analysis. Antimony(III), Sb^V, As^V and V^V do not interfere with the determination of Se. Tellurium is also reduced by 2-mercaptoethanol, but the elemental Te formed can be filtered off. The major interferent, As^III, can be oxidised with H₂O₂ to As^V prior to the analysis; the latter does not interfere with the determination of Se. The excess of H₂O₂ can be boiled off before the addition of 2-mercaptoethanol.