The electrochemical behaviour of 2,5-di-(-2-thienyl)-furan (SOS) on Pt has been studied in 0.2 M LiClO4 + acetonitrile + 0.25% (v/v) water solutions by cyclic voltammetry, chronoamperometry and chronopotentiometry. The monomer oxidises at more positive potentials than 0.7 V vs. Ag\AgCl giving a dark-blue film. The most reproducible film weights are obtained by flowing a constant current lower than 1.0 mA cm(-2). The resulting insoluble oxidised films are electroactive and electrochromic. Their control voltammograms exhibit two redox processes attributed to polaronic and bipolaronic states, together with an intermediate reduction shoulder ascribed to radical cation pairs. Electropolymerisation kinetics have been followed by ＇ex situ＇ ultramicrogravimetry, giving a maximum productivity of 1.85 mg mC(-1). The oxidised poly(SOS) contains polarons and electroinactive positive charges, both compensated with perchlorate counterions, The reduced poly(SOS) has been obtained in the same electrolyte bq applying a constant potential of 0 V to oxidised films. A partial electrodissolution of films is observed during their reduction. The electroinactive charges present in the resulting reduced stale cause its insolubility in the medium. Densities, conductivities and solubilities in different organic solvents of both oxidised and reduced poly(SOS) hale been determined. Short linear oligomers formed during SOS electropolymerisation have been detected in the saturated solutions of reduced poly(SOS) by mass spectrometry-fast atom bombardment.