Electrochemical and spectroelectrochemical behavior of regioregular monochlorosubstituted tetra(3-octylthiophene) (QT) has been investigated using UV-vis-NIR, EPR and Raman spectroscopies. The results obtained prove that QT undergoes dimerization to dichlorosubstituted octa(3-octylthiophene) (OT) and dynamic EPR spectroelectrochemical experiments combined with cyclic voltammetry can help to determine the mechanism of the process. UV-vis-NIR measurements made it possible to find electron spectra of the polaron and bipolaron formed during the oxidation of OT. The suggested mechanism of oxidation and electrohydrodimerization of QT into OT is corroborated by FT Raman spectroelectrochemical studies, which show significant changes in the position and intensities of the Raman bands coinciding with the polaronic and bipolaronic structures of the oligomers. These changes can he interpreted in terms of formation of a quinoid sequence of bonds in the oxidized oligomers.