We describe the application of a new model for the visualization of mobile species (ion and solvent) transfers accompanying redox switching of electroactive films. The system studied was polyaniline, for which the mobile species population changes were determined from redox-driven film mass changes using a thickness shear mode acoustic wave resonator. Acoustic admittance data were used to establish conditions under which the resonator frequency response could be interpreted gravimetrically. Charge and frequency changes accompanying the first redox transition of polyaniline films exposed to aqueous perchloric acid were then used to determine the film's ion and solvent populations. The data are best described by a mechanism in which the early stages of film oxidation are associated with proton transfer (exit) and the latter stages with perchlorate transfer (entry) to satisfy electroneutrality; solvent enters throughout film oxidation and exits throughout film reduction. The alternative of a single ion satisfying electroneutrality throughout would require the rather less likely situation of a non-monotonic solvent flux. Application of the model's diagnostic criteria indicates that the film solvent population is in equilibrium on the timescale of slow scan voltammetry, but shows thermodynamic non-idealities. Hysteresis in the film ion population signals failure of the redox state to maintain equilibrium with the applied potential. (c) 2006 Elsevier Ltd. All rights reserved.