The ion and solvent movement accompanying redox cycling of thin (similar to1 mum) films of poly(aniline)-poly(styrenesulfonate) were studied using the electrochemical quartz crystal microbalance and the transfer function method. The poly(aniline)-poly(styrenesulfonate) film was found to be acoustically rigid and smooth both during potentiodynamic deposition and during redox cycling in acid solution allowing the calculation of the changes in the mass directly from the variation in X-L. We find that during redox cycling in 1 M perchloric or hydrochloric acid electroneutrality of the poly(aniline)-poly(styrenesulfonate) is maintained exclusively by the movement of protons. The oxidation or reduction of the polymer and proton motion causes, on a slower timescale, the movement of water into the film on oxidation and out of the film on reduction. (C) 2004 Elsevier Ltd. All rights reserved.