for p(NDI-T2 P75) at all pH's explore in this study (suggesting superoxide formation is rate-limiting), if oxygen reduction on p(NDI-T2 P75) followed a serial 1+1 outer-sphere electron transfer pathway, we would expect a constant limiting current across the pH range (assuming that O2 solubility remains approximately the same). However, in addition to observing a pH dependence in the E1/2 for oxygen reduction in the RRDE experiments, we observe a decrease in the limiting current density with increasing pH. This observation is unlikely to be due to surface roughness as the limiting current is not dependent on the mass of p(NDI-T2 P75) on the electrode (Figure ). As demonstrated previously for metal-free carbon electrodes by Yang and McCreery, this observation can be explained through disproportionation of the superoxide intermediate to form O2 and hydro(gen) peroxide. The re-formed O2 can then be electrochemically reduced to superoxide generating a limiting current density that falls between the expected values from the Levich equation for an n = 1 and n = 2 electron transfer reaction, depending on the disproportionation rate. The disproportionation rate has been observed to scale with the concentration of hydroperoxyl, HO2, which decreases with increasing pH and thus lowers the observed limiting current density. We explore the possibility of disproportionation of superoxide following its formation from the outer-sphere reaction between the electron polaron in p(NDI-T2 P75) and oxygen through the use of a microkinetic simulation based on the following EC'D mechanism: