The hot corrosion of austenitic type 347 steel coated with alkali sulphates, namely Na 2 S0 4 , CaSO 4 , K 2 S0 4 and their mixtures, oxidised at 850°C in air has been investigated by measuring the oxidation kinetics. The formation of alkali iron trisulphate (Na,K) 3 Fe(S0 4 ) 3 seems to be responsible for the accelerated corrosion of the steel in a mixture of Na 2 SO 4 + K 2 S0 4 . Na 2 SO 4 + CaSO 4 or CaSO 4 + K 2 S0 4 form low melting eutectics which first attack the protective chromia scales followed by the formation of iron trisulphate as well as fluxing products. The scale morphologies as predicted from mechanistic considerations are in fairly good agreement with the observed morphologies.