Thermal plasmas may solve one of the biggest toxic waste disposal problems. The disposal of polychlorinated biphenyls (PCBs) is a long standing problem which will get worse in the coming years, when 180 000 tons of PCB-containing wastes are expected to accumulate in Europe (Hot ions break down toxic chemicals, New Scientist, 16 April 1987, p. 24.). The combustion of PCBs in ordinary incinerators (at temperature T similar to 1100 K, as measured near the inner wall of the combustion chamber (European Parliament and Council Directive on Incineration of Waste (COM/99/330), Europe energy, 543, Sept. 17, 1999, 1-23,)) can cause more problems than it solves, because highly toxic dioxins and dibenzofurans are formed if the combustion temperature is too low (T < 1400 K). The paper presents a thermodynamic consideration and comparative analysis of PCB decomposition processes in air or argon (+oxygen) thermal plasmas.