Surface enhanced infrared reflection- absorption spectroscopy with an attentuated total reflection configuration (ATR-SEIRAS) was used for the first time to identify the intermediates of the oxygen reduction reaction (ORR) on gold electrodes. Our study employed a Au thin-film electrode in acidic and alkaline solutions. In alkaline solutions, a potential dependent band at 1268 cm(-1), which we assigned to the antisymmetric bending mode of OOH of adsorbed HO2-, was observed between 0. 1 and -0.6 V versus Ag vertical bar AgCl, Cl-, exactly in the potential range where the ORR occurred. The assignment was supported by our isotope exchange experiment. The adsorbed HO2- is a reaction intermediate in the 4e(-) serial mechanism. In acidic solutions, there was only a very weak band at the same position, reflecting the fast protonation of HO2-. This finding may imply that the interaction between HO2- and An surfaces is very weak in acidic solutions, in agreement with the observed 2e(-) reduction mechanism.