Three new thiophene derivatives containing different activated ester groups covalently attached to the thiophene ring via a linear undecyl spacer have been synthesized. Electropolymerization of these monomers in acetonitrile led to stable electroactive polymers, while activated functional groups withstand the polymerization conditions and were correctly incorporated in the resulting polymers. These polymers show the characteristic electrochemical behavior of poly(3-alkylthiophene)s with the reversible redox transition in the range of 0.7-0.9 V. UV/Vis spectra of them exhibit an absorption maximum at around 460 nm. Conductivity measurements on oxidized films of these polymers by means of the two-probe method gave values in the range of 10(-3)-.10(-2) S cm(-1). The reactivity of the pendant activated ester groups was demonstrated by the reaction with butylamine, (4-amino-2-oxabutyl) ferrocene and 2-aminoanthraquinone. Spectroscopic studies and electrochemical characterization confirmed that the immobilized amino compounds are covalently bound to the polymer surfaces. These results suggest that the obtained new polymers can be used as electrically conducting carrier materials for the immobilization of biochemically interesting molecules.