A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the effects of film history and redox state of an unhydrated poly(o-toluidine) modified electrode immersed in aqueous perchloric acid (pH = 0) solution. The degree of film hydration affected the relative contributions of protons and anions exchanged during the first redox process and the potential of the first anodic voltammetric peak. A distinct proton contribution to the ion exchange during the first redox process was detected during the second and subsequent cycles. Electroneutrality during the second charge transfer step was maintained predominantly by proton expulsion. The determination of the diffusion coefficient of the exchanged species in the first redox process by the time dependence of laser beam deflection delay and by temporal convolution of the current signal are compared. The concomitant changes in the gravimetric and optical signals during film hydration demonstrate the participation of both proton and solvent during redox cycling of poly(o-toluidine).