When used at high temperature in air, titanium-based alloys form an oxide scale at their surface but also dissolve large amount of oxygen in their metallic matrix and this is a cause of embrittlement. Nitrogen is a secondary oxidant, which also dissolves in and embrittles the alloy. Oxidation experiments of Ti- 6Al-2Sn-4Zr-2Mo-0.1Si titanium-based alloy, for 10 0 0 h at 650 °C in synthetic air (N 2 -20%O 2 ) and in a mixture of Ar-20%O 2 , showed that nitrogen decreases both oxide scale growth and oxygen dissolution. Atom probe tomography was used to investigate the alloy/oxide interface. The results revealed that the nitrogen effect is due to the formation of interfacial layers of oxynitrides and nitride (Ti 2 N), but also to the formation of a nitrogen rich α-titanium-based solid solution, which all act as diffusion barriers for oxygen, because of their low oxygen solubility. A comparison between the experimental results and thermodynamic calculations is also reported.