Thin titanium nitride films were deposited by rf reactive sputtering. The N/Ti ratio varied between 0.88 and 1.12. The optical constants have been determined by ellipsometric measurements between 1.5 and 5 eV. The normal reflectivity was measured between 0.5 and 5.5 eV. The results have been fitted by a model dielectric function based on a set of Drude-Lorentz oscillators. In this way the contributions due to free carriers and to interband transitions could be unambiguously sorted out. The analysis of the optical properties sheds light on the nature of the stoichiometry-related defects of TiNx. It is shown that TiN contains one conduction electron per TiN unit, and that in TiN, the square of the plasma frequency varies in a manner indicating a loss of about one electron to each nitrogen interstitial and a gain of about one conduction electron for each nitrogen vacancy. In substoichiometric material the most significant defect is the nitrogen vacancy and in nitrogen-rich TiN, excess nitrogen behaves like an interstitial defect. The occasional presence of titanium defects is also detected.