We present ab initio calculations of the conformational effects produced by substitution of alpha,alpha'-bithiophene with fluorine, methoxy, methyl, or chlorine groups in one or both beta positions (T2X and T2X2). We find that the fluorine and methoxy substituents enhance the coplanarity of the rings in the trans conformation, while methyl and chlorine depress it by causing greater distortion. We also present the results of an atoms-in-molecules analysis of the molecular electron densities (bond and ring critical points). The C-alpha-C-alpha bond lengths and bond orders do not support the hypothesis that these conformational effects may be due to a g conjugation effect involving the substituents. On the other hand, we show that the planarization effects of the fluorine and methoxy substituents are mainly due to favorable intramolecular (SO)-O-... and (SF)-F-... nonbonded interactions. We also identify weak (CHO)-O-... and (CHF)-F-... interactions, stabilizing the planar cis states of the monosubstituted dimers. The changes in the X-C-beta-C-alpha and X-C-beta-C-beta' bond angles, indicating whether the X substituent distorts toward or away from the other ring in the planar conformations, are also in agreement with our interpretation.