The reactions of commercial polychlorobiphenyl mixtures of Sovol and Trikhlorbifenil brands (Russia) with sodium methoxide were studied in dimethyl sulfoxide and methanol. It was shown for the first time that Sovol congeners are exhaustively converted, without formation of potentially toxic products, in the reaction with alkali metal alkoxide based on a monobasic aliphatic alcohol. This process is a stage of chemical pretreatment of polychlorobiphenyls subject to destruction for the subsequent microbiological mineralization of the new derivatives.