Electropolymerization of thiophene was investigated in neutral and acidic medium from the standpoint of film formation and conductivity. Results obtained from voltammetric investigations have shown that proton affects both the electropolymerization of thiophene and its conductivity. This effect was explained by theoretical calculations using molecular mechanic (MM+, MM2) and semi-empirical (AM1) methods. From these calculations, it is seen that proton is significantly important in the stabilization and growth of the polymer. Proton is added to the sulfur atom in the polymer, which has a basic character because of its lone-pair electrons. With the addition of proton, the effect of the lone-pair electrons to the delocalization existing in the ring and the contribution to the conductivity of polythiophene is removed. Thus, the conjugation in the ring is reduced to the pi-system in carbon skeleton and the structure of the protonated polythiophene resembles to that of polyacetylene. (C) 2003 Elsevier Science B.V. All-rights reserved.