In this study, electrochemical properties of thiophene and polythiophene (PT) were investigated in tetrabutyl ammoniumperchlorate (Bt,NP), tetraethyl ammoniumtetrafluoroborate (Et4NBF4) and tetrabutyl ammoniumhexafluorophosphate (Bt(4)NPF(6)) supporting electrolytes in acetonitrile. Polarization curves and in situ conductance measurements show that PT films have different electrochemical properties and conductance values according to the type and concentration of the supporting electrolyte employed. In addition, the oxidation time, the potential and applied current were found to have a significant influence on the conductance values of the final polymers. A suitable mechanism is proposed. (C) 1999 Elsevier Science B.V. All rights reserved.