The vitrification of municipal waste incineration flyash has been achieved using an argon: hydrogen: oxygen plasma torch. Three different phases were obtained from the vitrication process: a vitreous phase, a condensate and a gas effluent. Thermic and chemical balances were evaluated, in order to optimize the process, also regarding environmental regulation constraints. The role of carbon, oxygen and hydrogen was studied from thermodynamic and kinetic simulations and experimentally verified. A special focus was put on the fate of chlorine during the vitrification. It was demonstrated that volatil metals combine with chlorine and play the role of halogen sink limiting the formation of phosgenics. Metal halogenides were recovered in the condensates.