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The oxidation of superalloys that rely upon the development ofAl 2 O 3 or Cr 2 O 3 scales for oxidation resistance in air containing water vapor are compared at temperatures from 700°C to 1100°C. It is shown that water vapor affects the oxidation of such alloys in different ways. Water vapor adversely affects the selective oxidation of aluminum in the -Al 2 O 3 -formers, and also affects the adherence of these oxide scales. The major effect of water vapor on the chromia-formers is enhanced vaporization of Cr 2 O 3 . Aluminide coated superalloys are also examined in these cyclic oxidation tests, and it will be shown that water vapor caused spalling of the -Al 2 O 3 scales, but the effect is small for strongly adherent -Al 2 O 3 scales.