Molecular orbital calculations have been performed on thiophene oligomers bearing heterocyclic substituents at the third position in order to understand the interplay of effects such as the nature of substituent, skeletal substitution pattern, heteroatom identity, etc. on electronic transitions. The geometry optimization was carried out using the semiempirical AMI method, and the electronic transitions predicted for the oligomers were extrapolated for polymers using an oligomeric approach. The experimentally determined optical transitions were found to follow the predicted trend of electronic transitions of monomers and polymers. © 2006 American Chemical Society.