4,5,6-Triaminopyrimidine reacts in acidic aqueous media with selenium (IV) to give a piazselenol which has an absorption maximum at 362 nm with a molar absorptivity of 1.72 x 10(4) 1.mole(-1).cm(-1). The compound is stable but not extractable into non-polar solvents. The calibration graph is linear up to 10 ppm of selenium, with a detection limit of 0.1 ppm in the sample solutions. Of the many different ions tested only iron (III) (in the presence of chloride) and tin (II) interfere. The method has good reproducibility, with a relative standard deviation of 1.5% for pure solutions. The application of this method to analysis of water and electrolytic copper is described.