Chlorine-induced high temperature corrosion is one of the major problems for structural materials used in incineration plants. The effect of chlorine on oxidation behaviour of four binary ferritic Fe–Cr alloys (Cr wt% = 15,20,25,30) was investigated in a simulated gas environment N2 –10CO2 -10H2O with and without 400ppm HCl at 650ºC for up to 290 hrs. The corrosion rate was determined from the weight gain kinetics and the corrosion products were analysed using optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and elemental dispersive spectroscopy (EDS). Iron-rich oxide scales formed on Fe-15Cr in both gases. Increasing Cr content, increased the tendency for Cr2O3 scale formation. The presence of chlorine accelerated the corrosion rate significantly. The corrosion mechanism proposed is “active oxidation”, caused by metal chloride vaporization and subsequent transformation of chlorides into oxides leading to the porous and less adhesive oxide layers observed. This mechanism was supported by microstructural analysis of reaction products together with reaction product thermodynamic analyses.