Overoxidation of polypyrrole (PPy) films has been studied with the aim of control their permeability towards ionic species in solution. During overoxidation electron-rich groups are introduced onto pyrrole units, and the conducting polymer is converted into an ion-exchange polymer. Differences in permeability are obtained by varying the component of the electrolyte solution in which the overoxidation of PPy is carried out. The exclusion of anionic species offers substantial improvement in the selectivity of voltammetric determination of cationic species. Prevention of electrode fouling due to protein and surfactant adsorption is observed in the case of overoxidized PPy film coated electrodes.