Owing to their combination of high hardness and increased fracture toughness, CrTaN coatings have recently gained increasing interest as suitable candidates for metal cutting applications. However, up to now, the detailed mechanisms underlying the oxidation of this promising coating system are not thoroughly understood. Thus within this work, the evolution of microstructure and phase composition of a cathodic arc evaporated Cr _0.74Ta _0.26N coating were illuminated in ambient atmosphere up to 1400 °C. In situ high-energy X-ray diffraction showed that powdered face-centered cubic (fcc) CrTaN displays an excellent oxidation resistance up to ∼1050 °C, where the formation of tetragonal (t) CrTaO ₄ and rhombohedral (r) Cr ₂O ₃ sets in. The compact CrTaN deposited on sapphire subjected to 1225 °C in ambient atmosphere exhibits intact fcc-CrTaN regions near the substrate, a porous intermediate layer of r-Cr ₂O ₃ and t-CrTaO ₄ and a dense r-Cr ₂O ₃ oxide scale at the surface.