The charge-transfer resistance, the redox capacitance and the diffusion coefficient of charge carriers at poly(o-phenylenediamine)-modified electrodes were determined by electrochemical impedance spectroscopy. Impedance results are interpreted in terms of the diffusion-migration transport of both electrons and protons through the film. Their coupled diffusion coefficients obtained at different potentials show a maximum near the formal potential of the polymer. This relationship between the oxidation level and the coupled diffusion coefficient indicates that the rate of charge transport within the film may be controlled by interchain electron-hopping. An exponential decrease in the coupled diffusion coefficient with increasing solution pH implies that the electron-hopping process is coupled with intermolecular proton exchange. Deprotonation of the polymer can result in a decrease in the homogeneous electron-transfer rate constant.