Polychlorinated biphenyls (PCBs) can be metabolized to reactive metabolites, such as PCB semiquinone radical anions (SQ(center dot-)), whose structure and role in PCB-induced toxicity are difficult to investigate due to their relative instability. The unrestricted LTB3LYP/6-311G** method was used to investigate several molecular descriptors of the syn- and anti-like conformation of SQs(center dot-). The bond lengths and angles of the quinone moiety of the SQs(center dot-) were in between the values reported for PCB quinones and hydroquinones, which is consistent with the distribution of the a highest occupied molecular orbital (alpha-HOMO). The dihedral angles between the two ring systems increased in the presence of ortho chlorine substituents and were smaller compared to the corresponding PCB quinones. The ground-state energies indicate that the anti-like conformation of the SQs(center dot-) is more favorable than the syn-like conformation. Molecular descriptor used for modeling of quantitative structure-activity relationships displayed some dependence on the conformation. These findings suggest that SOs center dot- in both the syn- and antilike conformation may interact differently writh target molecules, which may have implications for the toxicity of PCBs.