For the first time a joint theoretical and experimental method is proposed that allows one to elucidate the conformations of oligothiophenes in their respective ground and first excited singlet and triplet states. Semiempirical calculations (AM1 and ZINDO/S) are used to obtain the ground state conformation (S 0) of bithiophene and terthiophene. Similar potential energy curves are obtained for both molecules. A higher rotational barrier between adjacent rings is calculated for terthiophene suggesting that a higher conjugation length is effective for this molecule. This is confirmed by comparison with experiments. ZINDO/S is used to calculate the energy of the S 1 ← S 0 electronic transitions as well as the S 1 and T n excited state energies. These calculations suggest that, after excitation, both molecules relax to a planar S 1 conformation. The geometry of the molecules in their T 1 states is also predicted to be planar. From the S 1−T n energy gaps and the reported values of the intersystem crossing rate constants ( k isc) of bithiophene and terthiophene, it is suggested that the Franck-Condon factors are not playing an important role in the intersystem crossing probability of these thiophene oligomers.