The electrochemical reduction of oxygen on phenanthrenequinone-modified glassy carbon electrodes (GC) has been studied using a rotating ring-disk electrode (RRDE). Phenanthrenequinone (PQ) was covalently attached to the surface of GC by the electrochemical reduction of the corresponding diazonium salt. The redox potential of surface-bound PQ in 0.1 M KOH was 300 mV more positive than that of anthraquinone (AQ). The PQ modified electrode showed a much higher electrocatalytic activity for oxygen reduction. The results have been analysed using a surface redox catalytic cycle in which the key reactive intermediate is the semiquinone radical anion. The more positive redox potential of PQ compared with AQ and the larger value of the rate constant of reaction of the radical with O-2 result in an enhanced electrocatalytic activity of the functionalised electrode, producing H2O2 with a 100% yield. (C) 2003 Elsevier B.V. All rights reserved.