The authors present a comparative study of the properties of two solution-processible alkyl-substituted polythienylenes, poly(3-hexyl thienylene), P3HT and poly(3-dodecyl thienylene), P3DT. Both show the onset of pi - pi * interband absorption at 2 eV with the peak in absorption coefficient near 2.5 eV, and both show strong photoluminescence in an energy band extending from 1.3 to 2 eV. These properties are both due to intra-chain absorption across the pi - pi * gap, with the photoluminescence arising from the radiative decay of the singlet exciton. For the case of P3HT, photoinduced absorption measurements show the presence of two subgap absorption bands, which are associated with the formation of charged bipolarons which require charge separation between chains. P3DT, in contrast, shows a single induced absorption band, at 1 eV, which shows very different temperature and frequency dependence from that of the bipolaron bands in P3HT. The authors consider that a triplet-triplet transition of the triplet intrachain exciton is a possible assignment for this band, and consider that poorer contact between chains in P3DT over that in P3HT inhibits the transfer of charge between chains and thus prevents the formation of charged bipolarons in this polymer. They also discuss the role of the polymer structure in the stabilisation of the photogenerated bipolarons.