The high affinity of Ti-6Al-4 V (Ti64) powder to oxygen impacts the quality of the powder during additive manufacturing. The oxide scale consists of a double layer with α-Al₂O₃ in the outer part and TiO₂ in the inner one. Experimental and thermodynamic results are used to further understand the formation of these two oxide layers. Inert marker revealed the anionic growth of the duplex oxide scale. The thermodynamic results highlighted that interstitial aluminum enters and diffuses fast within TiO₂ (rutile) to form Al₂O₃ as the surface layer. Moreover, α-Al₂O₃ is considered as a non-stoichiometric oxide with a very small excess of oxygen as dumbbells at high partial pressures of oxygen. A remarkable effect of the impurities on the concentration of native point defects is observed in this phase. Finally, a reactional mechanism with elementary steps for the oxidation of Ti64 powder is proposed.