Owing to its excellent mechanical and thermal properties and outstanding oxidation resistance, TiSiN is used for protective hard coatings for cutting applications. While several reports confirm the oxidation stability of TiSiN up to temperatures above 800 °C, literature is currently lacking a thorough investigation of the oxidation sequence of this coating system. Thus, in this study the oxidation mechanism of TiSiN was monitored via in-situ synchrotron X-ray diffraction (XRD) and complemented by a detailed analysis of the microstructure and elemental composition of oxidized coatings. A TiSiN coating was deposited by cathodic-arc evaporation in an industrial scale deposition plant. In-situ synchrotron XRD experiments of the powdered coating showed an oxidation stability up to ~830 °C, followed by the formation of both, rutile and anatase TiO ₂ with increasing temperature. The formation of anatase during oxidation was confirmed by Raman and XRD investigations on a solid coating. High-resolution scanning transmission electron microscopy investigations revealed the oxidation of only several hundred nm of the coating surface after oxidation at 930 °C for 5 min, while increasing the temperature to 1130 °C resulted in full oxidation of the Ti(Si)N nanocrystals, accompanied by high porosity and significant grain coarsening. Furthermore, elemental analysis showed the presence of TiO ₂ grains surrounded by an amorphous Si-O-N phase as well as the formation of a TiO ₂ top layer due to diffusion of Ti to the surface. The obtained results provide detailed and novel insight into the oxidation mechanism of TiSiN as well as on the microstructure of oxidized TiSiN coatings.