The combustion of coal and Municipal waste generates very corrosive media particularly near the superheater tubes. Sulphur and chlorine are known to be the main corrosive agents in coal-fired plants and in waste incinerators respectively. This paper focuses on the study of the corrosivity of such environments composed of gases and ashes, at high temperature, on two commercial alloys with different chromium and nickel contents: the T91 and AC66 steels. Specific laboratory tests allowed to study the temperature effect and the influence of the ash and gas constituent contents (HCl-SO2-HO-K2SO4-Na2SO4-KCl-NaCl). Experimental results showed a higher corrosivity of the waste incinerator conditions compared to the coal-fired plant ones. Moreover the AC66 steel has a better corrosion resistance than the T91 alloy. Numerous factors are implicated in the corrosion processes and can act in concert.