The UV-Vis spectra of p(NDI-T2 P75) during reduction from its neutral to its bipolaron state in Ar-saturated 0.1 M KOH is presented in Figure . The increase in optical density at ~500 nm and the respective decrease near 720 nm correspond to electron polaron formation, and the increase in optical density near 630 nm is indicative of the electron bipolaron state. Figure presents the differential optical density of the electron polaron feature at 500 nm with respect to time and voltage. The appearance and disappearance of the electron polaron feature in Arsaturated electrolyte correspond well with the voltages observed in cyclic voltammetry. In O2-saturated electrolytes, however, the intensity of the electron polaron feature was notably suppressed and shifted to more cathodic voltages. These results suggest the rate of O2 reduction exceeds the rate of electron polaron formation in the film, where oxygen present in the electrolyte can rapidly consume the energetic electrons of the electron polaronic state of p(NDI-T2 P75) to generate hydro(gen) peroxide. Further evidence of this process is provided in Supplementary Figure in which we track the polaron feature at 500 nm as the electrolyte feed to the spectroelectrochemical flow cell was switched repeatedly between Ar-saturated and O2-saturated 0.1 M KCl electrolytes, resulting in the appearance and disappearance of the polaronic state, respectively. neutral, polaronic, and bipolaronic states of the polymer measured at characteristic voltages in Ar-saturated electrolyte, shown in Figure .