Cyclic and potentiostatic electrochemical quartz crystal microbalance measurements are reported for poly(aniline) deposited on Pt electrodes and immersed in aqueous solutions of HClO4, 4-toluenesulfonic acid and 5-sulfosalicylic acid. The effect of solution composition on the mass changes occurring simultaneously with electrochemical transformations of the polymer nias investigated. The results indicate that the pH and potential-dependent relative contribution of anions and cations to the overall ion exchange process is in connection with the degree of protonation of the reduced, partially and fully oxidised forms of the poly(aniline). The small mass change at lower potentials can be rationalised in terms of a model in which it is assumed that the expulsion of protons at the beginning of the first oxidation step and the incorporation of anions at a later stage of oxidation is basically determined by thermodynamics and not the kinetics of the system. Kinetic complications also arise for several reasons such as slow diffusion of ions and solvent molecules and relaxation of the polymer network. The actual film morphology depends on the nature of the solution used. It is proved that the pore size exclusion effect in the case of larger anions does not exist as thought previously, inasmuch as the originally compact structure formed in perchlorate solution can be transformed to a more open and swollen one under continuous cycling in the solution containing electrolyte of large ions.