An electrochemically activated glassy carbon electrode was employed for fabrication of an H2O2 sensor through incorporation of horseradish peroxidase (HRP) into the activated surface layer of the electrode during the electrochemical deposition of phenol. The immobilized HRP showed excellent stability and biocatalytic activity for the reduction of H2O2. The enzyme electrode exhibited linear responses to H2O2 in a concentration range of 5 x 10(-8) to 1 x 10(-5) M and its response time was less than 5 a. At a moderate operating potential of 0 V vs, SCE, the direct responses of H2O2 and other electroactive interferents, such as ascorbate and urate, on the activated glassy carbon electrode were minimal. The interference of ascorbate and urate was further reduced by the insulating polyphenol firm.