The structure and hot corrosion behaviour of cast nickel-based superalloy M38 irradiated with a CO 2 laser beam of 1.3 kW output have been studied using optical microscopy, X-ray diffraction, scanning electron microscopy and electron probe microanalysis and compared with the structure and hot corrosion behaviour of the unirradiated alloy. The hot corrosion tests were carried out in a crucible filled with 75 mol.% Na 2SO 4 plus 25 mol.% NaCl at 900°C for 15, 35, 55, 80 and 100 h. The test results showed that the irradiated alloy exhibited a hot corrosion resistance which was a factor of 2–3 better than that of the original unirradiated cast alloy. In contrast with the significant internal oxidation and sulphidation along grain boundaries in the unirradiated cast alloy, it was noted that, beneath a dense protective oxide scale consisting of mainly Cr 2O 3, there was no sign of internal oxidation and sulphidation in the irradiated alloy. The irradiated surface layer may be regarded as an effective coating which belongs to the superalloy itself.