In situ spectroscopic ellipsometry has been employed to determine the properties of titanium nitride (TiN) films during plasma-assisted atomic layer deposition by alternating TiCl4 precursor dosing and H-2-N-2 plasma exposure. Besides monitoring the film thickness when optimizing the half reactions, it is shown that spectroscopic ellipsometry is a very valuable tool for in situ studies of (air-sensitive) film properties such as resistivity, and for investigating the nucleation phase during initial film growth.) (c) 2005 American Vacuum Society.