Polythiophene films at the surface of electrodes containing ester groups are interesting potential carrier materials for reagents. Methyl thiophene-3-acetate (2) can be copolymerized with 3-methylthiophene (1) by means of cyclic voltammetry at potentials of 0-2.2 V. Higher potentials (0-2.4 V) lead to overoxidation of the copolymers. The ester groups were proved by FTIR spectra. Electrochemical investigations of 2,2＇-bithiophene (4) and 2 at equimolar ratios shows no successful copolymerization at potentials of 0-1.3 V. If the copolymerization experiments of 4 with 1 or 2 were carried out at molar ratios of 1:50 at 1.3 V 4 with its low oxidation potential can be polymerized without copolymerization of the other monomers. However, if the oxidation potential is increased stepwise arising from 1.3 V the oxidation of 1 or 2 takes part forming copolymers containing both the monomer components. HPLC investigations of solutions containing mixtures of 4 and 2 and also 4 and 1 in acetonitrile/TEABF(4) shows after exhaustive oxidation at a potential of 1.3 V the complete absence of 4. 1 and the ester 2 were not oxidized and copolymerized at this potentials. From the results of the copolymerization experiments as well as the HPLC investigations, it can be concluded that the dominant mechanism of the electrochemical polymerization is the radical cation dimerization.