Differential scanning calorimetry and thermogravimetric analyses combined with XRD and TEM investigations demonstrate that single-phase face-centred cubic Ta0.89Al0.11N1.2 coatings decompose into wurtzite AlN and hexagonal Ta2N phases upon vacuum annealing to 1500 °C. The Ta2N phase transformation proceeds via hexagonal Ta5N6 at ~1000 °C and hexagonal ε-TaN at ~1100 °C. A significant N2-loss between 1300 °C and 1500 °C, without a structural change of the Ta2N phase, suggests the formation of TaN0.3—at the Ta-rich Ta2N phase field.