Ti _1-xAl _xN hard coatings deposited by chemical vapor deposition (CVD) have attracted much attention recently due to their extraordinary nanolamellar microstructure and outstanding performance observed in metal cutting operations. Several published reports suggest further that CVD-Ti _1-xAl _xN exhibits an increased thermal stability and high temperature oxidation resistance when compared to state-of-the-art physical vapor deposited Ti _1-xAl _xN. However, the exact mechanisms underlying the oxidation of this coating system are not thoroughly understood yet. Thus within this work, the thermal stability and oxidation resistance of a powdered nanolamellar CVD-Ti _1-xAl _xN coating have been investigated at the synchrotron radiation facility applying a novel in-situ experimental approach. The sample was annealed in air between 100 and 1400 °C and 2D X-ray diffraction patterns were recorded simultaneously with the differential scanning calorimetric signal. The obtained diffraction data was successively analyzed using sequential Rietveld refinement, yielding the temperature-dependent phase composition. By combining this method with the differential scanning calorimetric data, it was possible to precisely track the onset and progress of chemical reactions. The results show that the different phases present in the sample oxidize individually, with the oxidation stability strongly depending on the Al-content. Further it was found that when Ti _1-xAl _xN spinodally decomposes in air, the formed TiN oxidizes directly after its formation while AlN retains its chemical stability. The present work provides not only a detailed insight into the thermal stability and oxidation resistance of CVD-Ti _1-xAl _xN but also proves the outstanding ability of the used method for analyzing metastable coatings systems.