The conformational analysis and molecular structure of 3,3＇-dimethylthio-2,2＇-bithiophene (DMS33BT), 3,4＇-dimethylthio-2,2＇-bithiophene (DMS34BT), and 3,4＇-dimethylthio-3＇,4-dimethyl-2,2＇-bithiophene (DMSDM34BT) were investigated by ab initio calculations at the Hartree-Fock level (HF/STO-3G and HF/3-21G*) as well as by semiempirical calculations [Austin Model 1 (AM1) and Parametric Method 3 (PM3)]. Ab initio calculations (HF/3-21G*) indicated that the insertion of two alkylthio chains in the 3,3＇ positions creates a sufficiently high steric hindrance to twist the molecule to a minimum conformation at 71.0 degrees land a second minimum at 120 degrees), with an energy barrier of 2.3 kcal mol(-1) compared with the coplanar trans (180 degrees) conformation. For this molecule (DMS33BT), one can see an absorption band characteristic of a twisted molecule with a large range of conformations. The fluorescence spectrum demonstrates that the molecule adopts, in the first singlet excited state, a more planar conformation. The presence of only one alkylthio substituent in the 3-position (DMS34BT) decreases the steric hindrance such that a more planar conformation (141.5 degrees, (HF/3-21G*)) is obtained, with a lower energy barrier of 0.5 kcal mol(-1). The insertion of an alkylthio and a methyl group in the 3,3＇ positions (DMSDM34BT) creates the maximum steric hindrance and the more twisted molecule (102.2 degrees), with the higher energy barrier compared with the planar conformation (5 kcal mol(-1)). It is concluded from these results that the steric hindrance created by an alkylthio group is less than that of a methyl or an ethyl group in the same positions. It is also shown that the ab initio HF/STO-3G and the semiemperical AM1 and PM3 calculations are not sophisticated enough to predict good energy minimum conformers and potential energy surfaces for these kinds of molecules.