New TiMgN decorative coatings were deposited by reactive magnetron sputtering on high speed steel substrates. Their optical properties and oxidation resistance were evaluated by spectroscopic ellipsometry. Increasing Mg content in TiMgN reduces the free electron concentration and hence lowers the plasma frequency. The related red-shift of the reflectivity edge in the visible spectrum gives rise to changing colours, from yellow through violet to grey. The oxidation resistance was evaluated by heating the samples in air at 400 degreesC for 14 h. The oxidation kinetics of the coatings was studied by measuring the change in ellipsometric angles Psi and Delta with oxidation time. It turns out that the Mg addition significantly improves the coating stability as compared to TiN. (C) 2003 Elsevier B.V. All rights reserved.