This paper presents a dynamic model for predicting the high temperature vaporization of an open complex chemical system with oxygen and chlorine in the gas phase. This model is based on the coupling of interfacial chemical equilibrium and mass transfer into a boundary layer of concentration. The model is applied to municipal wastes incinerator fly ashes which contains both oxides and chlorides, and compared with experimental data obtained with a transferred are laboratory plasma reactor. The analysis of the results indicates that the operation conditions play an important role for optimizing the thermal treatment of fly ashes, for example, in order to improve the stabilization elf heavy metals in the melt, high oxygen partial pressure in carrier gas is required. The vaporization behavior of preheated fly ashes from which the chlorides are eliminated is studied. The washing improves the stabilization of heavy metals in the vitreous phase. This last point and the main predictions of the model are compared with experimental results.