This paper investigates the bioxidant corrosion behaviour of CoNiCrAlY coated IN738 at 1100 °C. Free-standing CoNiCrAlY coatings and CoNiCrAlY coated IN738 were isothermally oxidised in air at 1100 °C for periods up to 100 h. It is found that the bioxidant corrosion occurred in the coated IN738 samples, in which the β phases in the CoNiCrAlY coating were significantly consumed by internal oxidation and internal nitridation. Aluminium nitrides and titanium nitrides were found to exist in the CoNiCrAlY coating after bioxidant corrosion. The internal nitridation is likely to occur when no protectives oxides can form at the coating surface.