The mechanism of the electroreduction of oxygen on An surfaces in basic media is examined using surface-enhanced Raman scattering (SERS) measurements and density functional theory (DFT) calculations. The spectroscopy reveals superoxide species as a reduction intermediate throughout the oxygen electroreduction, while no peroxide is detected. The spectroscopy also shows the presence of superoxide after the addition of hydrogen peroxide. The calculations show no effect of OH addition to the Au(100) surface with regard to O-O length. These results suggest that the four-electron reduction of O-2 on Au(100) in base arises from a disproportionation mechanism which is enhanced on An(100) relative to the other two low Miller index faces of Au.