Titanium was deposited onto silicon by electron beam evaporation in high vacuum. The Ti films were treated by plasma immersion ion implantation (PIII) with energetic nitrogen ions extracted from a plasma which was generated by electron cyclotron resonance microwave excitation. The elemental composition of the films was measured by Rutherford backscattering spectrometry and the different phases were identified by x-ray diffraction. The results show that nitrogen PIII treatment leads to nitrogen ion incorporation with nitride phase formation, accompanied by an increase in density. Two mechanisms for the incorporation of nitrogen can be distinguished : implantation and radiation enhanced diffusion.