Abstract: An experimental assembly has been set up, which was suitable for the investigation of high temperature corrosion in strong corrosive gases and the continuous weight gain measurement. The investigation of sulfidation behaviour of an Fe-25Cr ferrous alloy in H₂-H₂S mixed gases at 700~900℃ and 10^-1~10^-3Pa suphur partial pressure has been carried out. After an early slow transient period of sulfidation,the kinetics of Fe-25Cr alloy obeyed to different parabolic laws. The corrosion product comprised of an (FeCr)_1-xS(x≈ 0.24,21 at%~0 at%Cr) out layer of coarse grains and a thin (CrFe)S_x innerlayer of aquiaxial grains. The sulfidation procedure was controlled by the outwards diffusions of Fe, Cr cations.