Combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) measurements on short time scales following the application of a potential step are presented. The procedure for evaluating the exchange of mobile species (ions and solvent) is illustrated by application of the combined technique to a poly(o-toluidine) (POT) film redox switching in aqueous HClO4 solutions. A comparison of the predicted PBD profiles calculated from the current and mass responses to that of the measured PBD signal, by temporal convolution analysis, enabled the contributions of anion (ClO4 −) and solvent (H2O) transfer at the first redox step to be discriminated quantitatively. The results are consistent with a mechanism where anion exchange involves a counter flux of solvent, and suggest that a film volume constraint is applicable on the time scale of the experiment. During oxidation and reduction, the EQCM indicates a non-equilibrium movement of anions, confirmed by convolution analysis of the associated current and deflection signals.