The corrosion behaviour of SiC and Si3N4 prepared by chemical vapour deposition (CVD) technique was investigated in molten Na2SO4 salt under argon and argon-oxygen mixture gases. The CVD-SiC was more attacked than the CVD-Si3N4 in molten Na2SO4 salt. This was in good agreement with the results of the thermodynamic equilibrium calculations. The corrosion of both materials in argon-oxygen mixture was less severe compared to that in argon. This was attributed to the formation of amorphous SiO2 acting as a protective film against the corrosion. The apparent activation energies of the CVD-SiC and CVD-Si3N4 in molten Na2SO4 under argon gas were 167 kJ mol(-1) and 595 kJ mol(-1), respectively.