The determination of boron in water by use of ferroin has been studied, and a rapid method developed for the determination of 0·01 to 2·70 mg l–1 of boron. The boron is complexed with salicylate, and the ion-association complex of borodisalicylate with ferroin is extracted into chloroform. The intensity of the resulting colour is observed spectrophotometrically. The effects of ions likely to occur in sewage, river, drinking and sea waters were studied; only copper, zinc, carbonate and anionic detergents interfered significantly. Interference due to copper can be inhibited by the addition of EDTA, that due to zinc by the addition of diammonium hydrogen orthophosphate and that from carbonate by prior neutralisation of the sample. The interference of anionic detergents can be used to determine the concentration of such detergents in the same sample. The application of the method to sewage, sea, drinking and river waters is described.