The structures of poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-ethylenedithiathiophene) (PEDTT) vary only in the substituent chalcogen atoms, yet the electronic properties of the materials are surprisingly dissimilar. The difference in electronic band gaps is approximately 0.8 eV and the polymers behave very differently upon p-doping. Two new terthiophenes have been synthesised using Negishi coupling methods. The X-ray crystal structures of EDOT–EDTT–EDOT (OSO) and EDTT–EDOT–EDTT (SOS) show strong intramolecular chalcogen–chalcogen contacts which are responsible for persistent conformers in solution and solid state, although significant interchain interactions should also influence the properties of the materials. SOS and OSO can be polymerised by electrochemical oxidation to give the corresponding, well-defined poly(terthiophenes) PSOS and POSO. Spectroelectrochemical studies on all four polymers reveal strong similarities between PEDTT and PSOS, and between PEDOT and POSO. Together with independent electrochemical and absorption studies, the results indicate that the unique properties of PEDOT are influenced more by conformational effects (intrachain SO contacts) than substituent effects.