Previous work has demonstrated that this reaction proceeds through a hydroperoxyl, HO2, intermediate resulting in pH-dependent kinetics. For the homogeneous disproportionation of radiolytically produced O2 sphere 1-electron reduction to form superoxide followed by disproportionation. We summarize the energetics involved in the catalytic (inner-sphere) and non-catalytic (outersphere) pathways through a Pourbaix diagram (modified for the RHE scale) for the reactive oxygen species in Figure . When the ORR halfwave voltage (E1/2) for oxygen reduction is