We studied zirconium nitride layers prepared by reactive direct current (dc) magnetron sputtering and synthesized with nitrogen gas flow ranging from 1 to 9 sccm (standard centimeter cube per minute) N-2. We measured their electrical resistivity and recorded their X-ray diffraction patterns as well as their RBS spectra and optical reflectance curves. Thus we could determine their crystallographic structure, their nitrogen content and their optical properties by simulating the reflectance curves with Drude's model for stoichiometric and sub-stoichiometric samples, and an extended Drude model for over-stoichiometric samples. In this work, we focus on our stoichiometric sample S4, in order to compare the dielectric function as well as the optical indexes n and k, deduced from fit optical parameters, with those given in literature. There is a good agreement between these results. Besides, we determined the "optical resistivity" of our several samples and compared them with the "electrical resistivity" measured by a four-probe method. Also, a good agreement is found between both curves, which confirms that the formalism used to simulate the reflectance curves is well adapted to these compounds. (C) 2004 Elsevier B.V. All rights reserved.