The formation potential and pathway of less-chlorinated PCBs, including mono- and di-chlorinated biphenyls, was investigated when PCBs contained in transformer oil were dechlorinated by commercial treatment. Potassium hydroxide with polyethylene glycol (PEG-600) was used for dechlorination of PCBs, and its destruction efficiency to whole 209 congeners of PCBs was 97.1% on average, ranging from 95.6% to 98.2%. Homologue of mono-CBs showed the negative destruction efficiencies, while those of highly-chlorinated ones through tri- to deca-CBs showed better efficiencies than about 99%. In particular, mono-chlorinated biphenyl in meta-position (#2) was about 34-47 times enriched after dechlorination, thus the reactivity seemed to be in the order of ortho-, para-, and meta-chorinated biphenyl. Co-planar PCBs' dechlorination efficiency was averaged as high as 99.4%, but 3,4,4',5-TeCB (#81) showed a relatively lower efficiency than others' in one case. Not only sodium metal but also alkaline metal such as potassium was thought of affecting the formation or enrichment of mono-chlorinated biphenyls produced by Wrutz-Fittig reaction as intermediates and products.