A procedure for the spectrophotometric determination of trace amounts of phenols in natural and waste waters is proposed. The method is based on the reaction of the phenols with iodine monobromide, followed by extraction of the products with cyclohexane. Beer's law is obeyed in the range 8–160 p.p.b. for phenol, with a limit of detection of 1.1 p.p.b. without previous distillation. The method can also be used to determine p-substituted phenols and chlorophenols.