Three novel polythiophenes isomers are reported whereby the only difference in structure relates to the regiochemistry of the solubilsing sidechains on the backbone. This is demonstrated to have a significant impact on the optoelectronic properties of the polymers, and their pro-pensity to aggregate in solution. These differences are rationalized on the basis of differences in backbone torsion. The polymer with the largest effective conjugation length is demonstrated to exhibit the highest field effect mobility, with peak values up to 4.6 cm2 V-1 s-1 observed.