Surface degradation phenomena of two model equiatomic alloys from the CrMnFeCoNi alloy system were investigated in 2% O 2 and 10% H 2 O ( p O2  = 0.02 and 10 −7 atm, respectively) at 800 °C for times up to 96 h. The crystallographic structures, morphologies, and chemical compositions of the corrosion layers developing on CrMnFeCoNi and CrCoNi were comparatively analyzed by mass gain analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The oxidation resistance of CrMnFeCoNi is relatively poor due to the fast growth of porous Mn-oxide(s). CrCoNi forms an external chromia layer that is dense and continuous in a dry 2% O 2 atmosphere. This layer buckles and spalls off after exposure to 10% H 2 O atmosphere. Beneath the chromia layer, a Cr-depleted zone forms in the CrCoNi alloy in both environments. As the oxide scale spalls off in the H 2 O-containing atmosphere, a secondary chromia layer was observed and correspondingly enlarges the Cr-depleted zone. In contrast, as the chromia layer remains without significant spallation when CrCoNi is exposed to a dry oxidizing atmosphere, the region depleted in Cr is narrower. Graphic Abstract