A new series of regioregular poly(terthiophenes), bearing bis(thioether) side groups, has been prepared by electrochemical oxidation. Cyclic voltammetry of the parent trimers reveals an increase in electron donating ability in cyclic thioether derivatives. For example, one compound containing a central ethylenedithiothiophene (EDTT) unit gives two irreversible oxidation peaks which are significantly lower than the corresponding values for the structurally analogous bis(methylthio) derivative (ΔE1ox = 170 mV, ΔE2ox = 30 mV). The difference in oxidation potentials of each terthiophene in acetonitrile solution can be explained by examining the highest occupied molecular orbital (HOMO) energies. The electrochemical behaviour of the polymers illustrates the increase in p-doping ability of the EDTT-containing polymer. The cyclic voltammograms and electronic absorption spectra of the polymers show that the polymers containing cyclic thioether units have the lowest bandgap in the series (ca. 1.4 V and 1.5 eV, respectively). Photoinduced IR spectroscopy of poly(dithienyl-3,4-ethylenedithiothiophene) provides evidence of a long-living photoexcited charged state in the polymer.