Oxygen reduction at glassy carbon (GC) exhibits distinctively different voltammetric behavior at high (> 10) and low (< 10) pH. The peak potential is found to be around - 0.4 Vat pH 13, compared to - 0.6 Vat pH 7.4 as measured against a saturated calomel reference electrode. Using experimental voltammetry and numerical simulation, the difference in peak potential is interpreted in terms of a difference in reaction mechanism. At low pH, 02 reduction is evidenced to proceed via a solution phase pathway initially resulting in the reduction of oxygen to superoxide. Conversely, at higher pH, a different mechanism is favored involving the formation of a surface bound superoxide species. The switch between the two mechanisms is related to the protonation of the surface bound intermediate under less basic conditions.