In situ spectroscopic ellipsometry (SE) in the energy region from 1.5 to 5.5 eV was used to monitor the process of TiNx films deposited by dc reactive magnetron sputtering of Ti. The dielectric function ϵ(ω) measured by SE yields the optical response of TiNxfilms and valuable information on their structural characteristics that are also verified by electron microscopy observations. The plasma energy of TiNx was found to depend strongly on the stoichiometry of the material and therefore is a valuable parameter for in situ monitoring of TiNx stoichiometry. Analysis of the ϵ(ω) spectra of the TiNx films deposited in sequential layers by an effective medium theory provides precious information about the initial stages of growth, the composition, the deposition rate, and any change in film growth. © 1995 American Institute of Physics.