The aim of this work was to investigate the inter-relationships between the ion bombardment during film growth, the resulting chemical composition and microstructure and the optical properties of TiNx and ZrNx coatings. Samples were prepared by non-reactive d.c. unbalanced magnetron sputtering from TiN and ZrN targets. The energy and intensity of the argon ions impinging on the film surface was varied systematically using different bias voltages and different external magnetic fields applied via a Helmholtz coil system. Coatings were characterized using wave-length dispersive electron-probe microanalysis, X-ray diffraction, spectroscopic ellipsometry and CIE-L*a*b* colorimetry. For a brilliant appearance the formation of coatings with a well-defined crystal lattice and a chemical composition close to stoichiometry was found to be necessary. The metallic character of ZrN coatings characterized by the plasma frequency seems to be more pronounced than for TiN-based films. Varying ion bombardment conditions resulted in TiNx coatings with different chemical compositions (x ranging from 0.85 to 1.02). Thus, also their optical properties were determined by their chemical composition. For ZrN, coatings, the lattice parameter was affected by the ion bombardment for given values for the ion intensity, giving rise to a close relationship to the coloration. (C) 1998 Elsevier Science S.A. All rights reserved.