Titanium silicide Ti5Si3 has been introduced in TiN coatings by in situ chemical vapor deposition (CVD) in order to improve their oxidation resistance. The behaviors of these TiNSi composites and of pure TiN in air have been compared in the range 800-1150 degrees C. Oxidation kinetics have been deduced from in situ thickness measurements of the oxidation layers as a function of time, by means of high-temperature X-ray diffractometry. Parabolic time dependence for the TiN oxidation with an activation energy of 187 kJ mol(-1) and a pre-exponential factor of 1.4 10(-3) cm(2) s(-1) has been found in agreement with results previously reported. Concerning the TiNSi, two regimes of oxidation have been evidenced. Above 950 degrees C, the oxidation layers grow parabolically with time according to a.diffusion-limited process. Below 950 degrees C, a fast oxidation occurs in a first stage. Then, the higher the temperature, the faster a threshold is reached corresponding to a complete passivation. Taking into account SIMS and XPS analyses in addition with TEM and EDS information, some assumptions have been proposed to explain the TiNSi oxidation mechanism,