An electrochemical system has been devised to measure phenol concentrations in aqueous solutions. The unit employs the immobilized enzyme, tyrosinase, to oxidize phenol in the presence of saturating levels of oxygen. The oxidation product, ortho-benzoquinone, is then chemically reduced in the presence of an excess of ferrocyanide ions. The coupled oxidation of ferrocyanide ions to ferricyanide ions results in a measurable potential difference in the electrochemical system. The resulting zero current potentials in these steady-state potentiometric measurements are shown to be directly proportional to the logarithm of phenol concentration over the range of 3.8 × 10-7 to 1 × 10-4 m. The results of studies carried out with alternate substrates for the enzyme and interfering compounds are also presented.