Determination of the phenol index in waters of different origin (such as ground waters, surface waters, seep waters and waste waters) in mass concentrations of 0.01 to 1 mg/l (in undiluted sample). In particular cases, the range of application may be adapted by varying the operating conditions. Clause 3 describes the determination of phenol index (without distillation) after extraction, and in clause 4 the determination of phenol index (without extraction) after distillation is given.