O-2 reduction on Hg adatom-modified Au electrodes has been examined in acidic, neutral and alkaline solutions using cyclic voltammetry and the rotating ring-disk electrode technique. The half-wave potential for the O-2 reduction in 0.1 M NaOH solution was 0.15-0.2 V more positive on the Hg adatom-modified Au electrode than on the ban Au electrode. That is, the O-2 reduction was accelerated on the Hg adatom-modified Au electrode surface in the potential range from ca. -0.1 to -0.5 V, compared with the bare Au electrode. Moreover, the accelerated reduction of hydrogen prl oxide (HO2) to OH could be also observed in the same potential range. Thus, it was found that the O-2 reduction on the Hg adatom-modified Au electrode proceeds with the exchange of four electrons in a series of pathways leading to the formation of OH [O-2 --> HO2---> OH-] in the potential region from ca. -0.1 to -0.5 V. In the potential region from -0.3 to -0.5 V, the reduction current of O-2 decreased from the diffusion limiting current corresponding to four-electron reduction of O-2 to Chat for two-electron reduction of O-2. The efficiency for the four-electron reduction of O-2 depended on the surface coverage (theta) of Hg on the Au electrode surface, and the highest efficiency was obtained on the electrode with theta = 0.24. The four-electron reduction current of O-2 decreased with decreasing pH of the solution. The reduction of HO2- to OH- ion and also its oxidation to O-2 in alkaline media were found to be accelerated on the Hg adatom-modified Au electrode. The observed acceleration of the reduction of O-2 is explained satisfactorily by taking account of the potential dependent adsorption of OH- on the Hg adatom-modified Au electrode.