Electrochemical impedance spectroscopy yields significant information concerning oxygen reduction because the time constants in the reduction process can be discriminated. In the present paper, the theoretical equations of the electrochemical impedance for the various oxygen reduction pathways are derived. In the cases of the two-electron and direct four-electron reductions, the electrochemical impedance simply shows one capacitive semicircle on the complex plane. When the oxygen is reduced by an indirect four-electron reaction or a two-electron reduction followed by a disproportionation reaction, the complex plane plot of the electrochemical impedance shows two semicircles. In this case, the semicircle in the low frequency range is related to the time constant of adsorption and desorption of the reaction intermediate involved in the consecutive reactions. The conditions that the electrochemical impedance shows an inductive semicircle in the low frequency range are discussed on the basis of numerical simulations. The channel flow double electrode was used to measure the j - E curve and electrochemical impedance of glassy carbon, Pt and Ag electrodes at oxygen reduction potentials in alkaline solution. The complex plane plots of the electrochemical impedance of Ag and Pt electrodes show an apparent inductive semicircle, indicating the presence of the reaction intermediate HO2-. (C) 2003 Elsevier B.V. All rights reserved.