AbstractThe isothermal oxidation of binary and ternary, Nh‐containing titanium aluminides was studied at 900°C in air and Ar/O2. It was found that the presence of nitrogen in the atmosphere is in some cases beneficial, in others detrimental. For an interpretation of the effects of nitrogen it is necessary to consider the various oxidation stages in which the mechanisms of titanium aluminide oxidation fundamentally differ. For High‐Al alloys, which tend to form protective alumina based scales in the early oxidation stages, nitrogen always scenes to he detrimental because it promotes the formation of a more rapidly growing mixed alumina/titania scale. After this shift to mixed oxide scales, nitrogen can be detrimental as well as beneficial. It is proposed that the last mentioned case prevails if a near‐continuous Ti‐rich nitride layer is stable at the oxide/alloy interface, thereby preventing internal oxidation of aluminium. This beneficial effect, of nitrogen was found under all studied conditions for the Nb‐containing alloys but only in some cases for the bin