A theoretical investigation of the electronic structure and optical and charge-transport properties of polythiophene chains incorporating thienothiophene units is reported. Such polymers exhibit a better stability and, in some cases, a larger hole mobility than poly-3-hexylthiophene (P3HT). Quantum-chemical calculations have been performed on oligomers of increasing chain length to establish the changes in the electronic and optical properties when going from P3HT chains to the new derivatives. We have also estimated important molecular parameters governing charge transport in organic semiconductors (i.e., internal reorganization energies and transfer integrals) to determine whether the molecular structural changes along the polymer backbones are likely to be responsible for the increase in the hole mobility. © 2007 American Chemical Society.