Electrochemical formation of self-doped sulfonated polyaniline (SPAN) was carried out in acetonitrile-containing anhydrous fluorosulfonic acid (FSO3H). Fluorosulfonic acid was used as both sulfonation reagent for aniline and also as supporting electrolyte. The degree of sulfonation of the polymer was controlled by varying FSO3H and aniline concentrations while the film is being deposited. Different dry conductivity values for the polymer (1.24-14.6 S/cm) were measured depending on the degree of sulfonation of polyaniline. Polychromic behavior of SPAN (transparent yellow-green-dark blue) was observed when the cyclic voltammograms were taken between -0.30 and +1.90 V (vs. Ag/AgCl, saturated) during the growth of polyaniline film. The polymers having different degrees of sulfonation were found to be soluble in basic aqueous solution, dimethyl sulfoxide (DMSO) and N-methylpyrrolidone (NMP). SPAN was characterized by elemental analysis, FT-IR spectroscopy, UV-visible spectroscopy and cyclic voltammetry. (C) 2002 Elsevier Science B.V. All rights reserved.