The in situ electrochemical–ESR measurements of poly(aniline-co-o-aminophenol), PAoA, showed a strong dependence of the ESR signal intensity on the applied potential, revealed an unusual phenomenon that the ESR signal of PAoA can exist in a wide potential range from −0.20 to 1.30 V in aqueous solutions from pH <1 to 5.0, and gave a conversion potential of polaron to bipolaron, depending on pH and buffer capacity of the solutions. The ESR measurements also demonstrated that the ESR signal intensity of PAoA decreased with increasing pH, accompanied by increasing peak-to-peak line width; however, the g value of PAoA was hardly affected by pH and the applied potential. PAoA was first used to test the influence of antioxidants on its ESR spectra, which demonstrated that the ESR signal intensity of PAoA in the presence of dopamine and catechol was affected slightly; however, the ESR signal intensity of PAoA in the presence of ascorbic acid decreased pronouncedly compared to that of PAoA in its absence. Therefor...