The ion exchange mechanism of chemically sulphonated polyaniline (SPAN) has been investigated by means of probe beam deflection and quartz crystal microbalance techniques in aqueous and non-aqueous electrolytes. Protons are predominantly expelled during the first and second oxidation step in acidic aqueous solution, this expulsion being accompanied by a counterflux of solvent. A minor (10%) exchange of anions cannot be excluded. There is no exchange of alkaline metal cations in acidic (pH 1) solutions of salts. In non-aqueous electrolyte, expulsion of cations seems to be the dominant flux during SPAN oxidation, while solvent counterflux plays a significant role. The ion exchange properties of SPAN are favourable for applications such as cation transfer batteries or pH actuators.