On a glassy carbon (GC) surface covered with a covalently bonded methylphenyl monolayer, O-2 reduction to superoxide was observed and shown to be chemically reversible above pH 10. The voltammetry is completely explained by electron tunneling through the organic monolayer, then degradation of O-2(.-) in aqueous solution by known homogeneous mechanisms. As the pn is decreased below about 10, O-2(.-) in solution decays to O-2 and H2O2 by routes previously deduced from pulse radiolysis experiments. In contrast, a GC surface cleaned with isopropanol and activated carbon is very active toward adsorption, and a two electron reduction to peroxide is observed. An adsorbed intermediate is proposed to be a surface hydroperoxide analogous to stable organic peroxides of the general formula ROOH. On clean, unmodified GC, the pH dependence of the O-2 reduction mechanism is consistent with control of the reduction process by adsorbed O-2(.-) or (O2H)-O-.. In the absence of adsorption sites on the carbon surface, degradation of electrogenerated superoxide occurs entirely in solution.