Abstract: Thermodynamic analysis revealed that the presence of gaseous SO3 and H2O in the melting process of ash from municipal solid waste incineration leads to a reaction to form Na2SO4 and evolves HCl which is required for the chlorination-evaporation reaction with NaCl as a chloridizing agent at the comparatively low temperature from 800 to 1000°C. Moreover, the activity of Na2O is greatly lowered by adding Al2O3 to Na2O-SiO2 molten slag phase, thus boosting the redecomposition reaction of Na2SO4 to Na2O and SO3 through the reaction with slag phase at higher temperature zone. The residues of highly chemical stability can therefore be obtained.