The present invention relates to a process for preparing polythiophenes comprised of unit structures of the general formula 1: ##STR1##

where R₁ and R₂ independently represent hydrogen or a C₁C₄ alkyl group, or together represent an optionally substituted C₁C₄ alkylene group, preferably an optionally alkyl-substituted methylene group, an optionally C₁C₄ alkyl- or phenyl-substituted 1,2-ethylene group, a 1,3-propylene group or a 1,2-cyclohexylene group. More specifically, the process according to the present invention is characterized by the fact that it is performed by cationic polymerization from 2,5-dihalothiophene in the presence of an acid catalyst, such as Lewis acid, protic acid, oxygen acid, or polymeric acid. The conductivity of the resulting polythiophene is 255 S/cm under optimal conditions.