Cyclic voltammetry (cv) experiments at Pt electrodes were carried out to produce copoly(3-methyl-2,5-thiophenediyl-2,5-thiophenediyl-3-acetic acid) (3a) from the monomers 3-methylthiophene (1) and thiophene-3-acetic-acetic acid (2a), and to study the mechanism of electrocopolymerization. It was found by means of esterification and amidation and further chemical reactions of the copolymers thus obtained that the COOH groups are covalently bound to the copolymer structure. The charge/discharge reversibility and the overoxidation (irreversible redox process occurring when the covered electrode is scanned beyond the positive potential limit) of 3a and some of its derivatives are demonstrated by representative cyclic voltammograms. In a series of copolymerization experiments the positive potential limit (E(pos)) was lowered step by step from 2.20 to 1.70 V (vs. s Ag/AgCl) below the potential for the oxidation of 2a (E(mox) = 2.11 V vs. s Ag/AgCl) and, yet, the copolymer 3a was obtained. The results of the examination are discussed with the aid of data from cv, MS, FTIR-GIR (Fourier-Transform-Infrared-Grazing Incidence Infrared) spectroscopy, H-1 NMR/C-13 NMR spectroscopy, and elementary analyses.