Electrochemical studies of the [alkene, H2O\(Pd + VGCF) anode\H3PO4/silica-wool\VGCF-cathode\O-2, NO] cell were carried out at 373 K (VGCF, vapor-grown carbon fiber). The oxidation rates of C2H4 and C3H6, i.e., current densities, were increased more than ten times by the addition of NO to the O-2 stream in the cathode. A carbon electrode (VGCF-electrode) without precious metals could function as the cathode for the reduction of the mixture of O-2 and NO as well as a Pt cathode. The cyclic voltammetry and electrode potential studies over the VGCF-cathode in O-2, NO, O-2 + NO, and NO2 suggested that NO2 produced from O-2 and NO in the gas phase worked as a strong oxidant. NO2 easily reduced to H2O and NO that was re-oxidized to NO2 by O-2 in the cathode. NO functioned as a catalyst for the electrochemical reduction of O-2 to H2O. The electrochemical studies over a Pd electrode suggested that the oxidation state of the Pd anode changed from Pd-0 to Pd2+ under short-circuit conditions. The dramatic changes in product selectivities and formation rates in the C3H6 oxidation were due to the change of the oxidation state of the Pd anode by the addition of NO to the O-2 stream. The electrochemical oxidation of alkene proceeded over the Pd2+ anode, but not through a stoichiometric oxidation with H2O by Pd2+ (the Wacker oxidation mechanism). (C) 2003 The Electrochemical Society.