The thermal stability and composition of the gas phase of the most common technical mixtures of polychlorinated biphenyls of the Sovol and Trikhlorbifenil brands have been studied by simultaneous thermal analysis and mass spectrometry in comparison with their derivatives synthesized by the reaction of these mixtures with sodium methoxide in a medium of dimethyl sulfoxide and methanol. It has been determined for the first time that derivatives of polychlorinated biphenyls are less thermally stable compounds in comparison with the initial substances and, in pyrolysis, undergo not only evaporation but also decomposition to the simplest volatile species. The thermal destruction of the derivatives requires lower energy consumption and is characterized by lower emissions of hazardous chemicals.