In most thermal waste treatment processes, the environmental concern lies in toxic metals and dioxin emission. Regulations have been issued to reduce the amount of dioxin emitted through gaseous effluents. Dioxins have also been found in fly ash and at a much lower extend in bottom ash. In this work the behavior of dioxins is considered in the context of a fly ash plasma treatment developed in order to render the flyash inert and to reduce its volume. The composition of the flyash has been measured and the chemical balance on chlorine has been established.