3-Chlorothiophene (CT) was electrochemically polymerized in mixed electrolytes of a boron trifluoride diethyl etherate solution containing 0-20% (by volume) sulfuric acid. The oxidation potentials of the monomer in these media were measured to be only 1.06-1.31 V (vs Ag/AgCl). These values were much lower than that of CT in acetonitrile and 0.1 mol/L (Bu)(4)NBF4 (1.92 V vs Ag/AgCl). Poly(3-chlorothiophene) (PCT) films with conductivities of 0.1-2 S cm(-1) were obtained. The structure, morphology, and electrochemical behavior of the PCT films also were investigated.