O-2 reduction has been performed at Au nanoparticle-electrodeposited Pt electrodes (Au/Pt electrodes) in O-2-saturated 0.1 M KOH solution. Cyclic voltammetry (CV) and rotating ring-disk electrode (RRDE) techniques have been used in this investigation. Two reduction peaks were observed on the CV for the O-2 reduction at the Au/Pt electrodes. The reduction pathway of O-2 to either HO2- or OH- is highly dependent on the electrode potential and the Au surface coverage of the Pt electrode as well. The Au/Pt electrodes exhibit different electrocatalytic behaviour in three potential regions: At E > -300 mV, the 2-electron reduction of O-2 to HO2- predominates, at -300 > E > -500 mV, the 4-electron reduction predominates, and at E < -500 mV, the 2-electron and 4-electron reduction pathways proceed concurrently. The maximum production of HO2- (as a major reduction product) was obtained at a potential of ca. -240 mV versus Ag\AgC\KCl(sat), which is more positive by ca. 250 mV than the corresponding value at the bulk Au electrode (i.e. -500 mV vs. Ag\AgC\ KCl(sat)). The increase of the surface coverage of Au (i.e. increase of deposition time) on the Pt electrodes leads to a loss of this activity towards the O-2 reduction to HO2-. At a relatively higher negative potential, i.e. at -400 mV, the 4-electron reduction of O-2 to OH- occurs predominantly at all the examined Au/Pt electrodes with different Au loadings. Further negative increase of the potential (more negative than -400 mV) results in a partial contribution of the 2-electron reduction of O-2 with the 4-electron reduction. This behaviour could be understood in view of the potential dependence of the amount and the nature of the adsorbed species on the Au/Pt disk electrode surface. (C) 2003 Elsevier Science B.V. All rights reserved.