We report on spectro-ellipsomettic investigations of reactively sputtered tantalum nitride (TaNx/SiO2) and vanadium nitride (VN/Si) films. Several TaNx layers with different deposition temperatures (200-400 degreesC) and a VN layer were prepared optically thick and analyzed using the bulk formula. For thin TaNx layers prepared with the same temperatures as thick layers, the Drude-Lorentz parameterization of dielectric functions was used by simultaneous fitting of ellipsometric and optical transmittance spectra. VN pseudo-dielectric functions show strong metallic character, which is typical for other transition metal nitrides. In contrast, the TaN, dielectric constants obtained here range from a metallic to a non-metallic character depending on substrate temperature, but independent of the film thickness. This substrate temperature dependence may be due to multiple phases in the TaN, films. (C) 2003 Elsevier B.V. All rights reserved.