In an electrochemical environment, an O2 in its triplet ground state could capture an electron from the electrode by an outer sphere charge transfer, producing an O2 -which is stabilized by polar solvents and brought to the electrode surface as a chemisorbed O2*. On Au(100), such a reaction step has been analyzed theoretically by Schmickler and coworkers and identified as the RDS for ORR in alkaline solution. Experimentally, there are now convincing evidences for chemisorbed O2*, usually as superoxide, and also for HOO* and H2O2* on gold electrode surfaces, based on in situ vibrational spectroscopic studies, either by Surface Enhanced Raman Scattering or Surface Enhanced Infrared Reflection/Absorption. In alkaline aqueous solution, the O-O stretching feature for adsorbed superoxide was observed at 1130 or 1150 cm -1 . A similar feature was also observed in O2 saturated DMSO solution, which was definitely due to O2*, rather than HOO* or H2O2*, since the highly polar DMSO solvent was aprotic. We shall later discuss the kinetic implication of such an outer sphere step, after mapping out the barriers for the elementary reactions following O2 chemisorption on Au(100) in alkaline solution and in an electrochemical environment.