We describe electrochemical quartz crystal microbalance measurements on poly(o-toluidine) (PoT) films exposed to aqueous perchloric acid solutions (pH 0 to 2). Crystal impedance measurements show the films to behave rigidly. PoT redox snitching occurs in two stages and is accompanied by nonmonotonic mass changes that are the result of perchlorate counterion, proton co-ion, and solvent transfers. The extent and rate of each of these transfers are dependent upon electrolyte concentration, experimental time scale, and the switching potential, so that observations in a single electrolyte on a fixed time scale cannot be unambiguously interpreted. In the first and second oxidation stages, respectively, electroneutrality is satisfied predominantly by anion entry and proton exit. Throughout, there is solvent expulsion, to the extent of up to three water molecules per redox site converted. Transient experiments allow partial separation of the total mass change into individual species contributions. As the electrolyte concentration is decreased, there is progressive failure to establish first solvation, then co-ion transfer, then redox (electron/counterion transfer) equilibria.