In situ sulfonation reaction of aniline was carried out in water and water-acetonitrile mixture by changing both aniline and fluorosulfonic acid (FSO3H) concentrations. The optimum conditions for the polymer formation was determined. The dry conductivity values of the film increased and the solubilities of the polymer decreased with increasing volume of acetonitrile in the mixture. The polymer film was found to grow much faster compared to the growth in non-aqueous acetonitrile. FTIR, elemental analysis, cyclic voltammetry and UV-VIS spectroscopic methods were used to characterize the polymers. (C) 2002 Published by Elsevier Science B.V.