Poly(3,4-ethylenedioxythiophene), PEDOT, films were electrochemically prepared from aqueous solutions containing different supporting electrolytes (ClO4-, SO42- and H2PO4-). Characterization of the resulting films was carried out using cyclic voltammetry and potential step experiments combined with the dissipative quartz crystal microbalance technique. The ion incorporation caused an increase of the film mass and below 10(-2) M depended on the concentration of the solutions. Potential step experiments showed that the pH of the solutions also has an influence on the mass change of the films, which can only be partially attributed to the pH dependence of the charge of the anions. In principle, the ratio of the mass and the passed electric charge (apparent molar mass) during the redox switching of the polymer can be used to identify the ionic species. (c) 2007 Elsevier Ltd. All rights reserved.