Abstract: High temperature corrosion behaviors of alloy Co-30Cr, Co-30Cr-4A1 and Co-25Cr-loNi-4Al-5Ta-0.5Y has been studied at the presence of Na₂SO₄-K₂SO₄ salts mixture. The experimental results showed the internal sulfidation-internal oxidation to be the major mode of attack. Much severe attack is found in alloy Co-30Cr, and sulfide rich in Cr developed deep in the alloy matrix. Fine inner oxide precipitates rich in Al is found in alloy Co-30Cr-4Al, and sulfur penetration is detected by S radioisotope method. Little inner oxide precipitates rich in Al is found in alloy associated with Co-25Cr-10Ni-4Al-5Ta-0.5Y.