Chirality has been used as an internal probe of structural changes accompanying micro-aggregation of a soluble regioregular poly(alkylthiophene). First, the synthesis of new polythiophenes having long alkyl chains is described. The UV/vis and circular dichroic spectroscopy are shown to indicate that the transformation of a disordered species in solution into an ordered one in a micro-aggregated state is a two-step process. Tentative proposals are made about the nature of the intermediate state, which, however, remains unclear.