The kinetics of oxygen reduction at a gold electrode was studied in 0.5 M sulfuric acid, in which different kinds of straight-chain [CF3(CF2)(2)CH2OH, CF3CF2CH2OH, and CF3CH2OH] and branched [(CF3)(2)CHOH] fluorinated alcohols were added. The adsorbed layers of the fluorinated alcohols were used as models of the fluorocarbon phase of the perfluorinated polymer electrolyte in gas-diffusion electrodes in proton-exchange membrane fuel cells. A rotating ring-disk electrode was used to determine kinetic parameters for O-2 reduction and to detect intermediate H2O2 formation. The kinetics of oxygen reduction were strongly dependent on the molecular structure of fluorinated additives. The addition of the straight-chain fluorinated alcohols enhanced the kinetic current density while addition of the branched alcohol did not. The linear C-3 fluorinated alcohol, CF3CF2CH2OH, gave the maximum enhancement effect. Oxygen is reduced predominantly via the two-electron series path in the range of 0.4 to 0.0 V at Au, on which no effect of fluorinated additives was observed. The rate constant for intermediate H2O2 reduction, k(3), was negligible in the range 0.40-0.25 V, whereas it increased with decreasing E-D in the range 0.25-0.0 V. In the lower potential range, k(3) decreased with an increase in the concentration of fluorinated alcohol and this decreasing tendency was greatly dependent on the molecular structure of the fluorinated alcohol.