The room temperature oxidation process of thin TiNx films, grown by DC reactive magnetron sputtering, was studied by in-situ and real time spectroscopic ellipsometry, X-ray reflectivity, Auger electron spectroscopy, X-ray photoelectron spectroscopy and stress measurements. The films were deposited at various negative bias voltages (V-b) in order to vary the TiNx stoichiometry and then exposed to several gases and to air at room temperature. The oxidation rate was found to depend strongly on V-b and thus on film stoichiometry. Fast oxidation was observed in films deposited at low \V-b\, that contain nitrogen which is weakly bonded to Ti and can be easily replaced by oxygen. At high \V-b\ values (aboue - 80 V) stoichiometric TiN films were developed, that were stable and not prone to oxidation. The results showed that oxidation takes place in the bulk of thr: film and can proceed up to about 10% transformation of titanium nitride into titanium oxide. The fastest step in the him oxidation mechanism is diffusion of oxygen along the columnar grain boundary structure, followed by the reaction of oxygen with the weakly bonded TiN. The results suggest that the oxidation process proceeds with different stages involving formation of TiON, followed by a slow stage of TiO2 formation. These microstructural changes are consistent with the evolution of compressive stresses in the films surd the increase in film density and thickness and the: decrease in surface roughness as determined by X-ray reflectivity.