The diffusion of nitrogen into Ti silicide films allows the performance of complementary metal oxide semiconductor (CMOS) components to be improved. In this work, the thermochemical treatment is carried out in an expanding microwave plasma reactor using (Ar-33%N2-1%H2) gas mixtures. This process promotes the chemical reactions on the surface of metals. The diffusion of nitrogen into the film is improved by the reducing effect of NHx and/or H species towards passive layers such as oxides which form a barrier of diffusion in the surface layers during the process. The simultaneous formation of Ti nitrides and silicides at the surface and at the film-substrate interface, respectively gives rise to two competing processes which result in the growth of the Ti nitride phase at the expense of the Ti silicide phase at a critical temperature of 800°C. This paper reports on a comprehensive analysis of the evolution of TiSi2 and TiN phases and microstructural properties of films by means of X-ray diffraction, Raman spectroscopy, transmission electron microscopy and selected area electron diffraction investigations. Square shaped crystals of TiN are identified on the top of round shaped crystals of TiSi2. The growth of the TiN phase at the expense of TiSi2 induces a catastrophic decrease of the intensity of the (040) diffraction line of TiSi2 and a huge increase of the (220) reflection line of TiN. The microstructural properties changes during the process such as the formation of TiN crystals of nanometric size in the bulk of the TiSi2 phase as well as the migration of free Si which epitaxially grows at the film-substrate interface have been evidenced by very detailed investigations for the first time. The results are related to the mechanism of formation of TiN from the reaction between TiSi2 and nitrogen.