In this study, the effect of nitrogen pressure, bias voltage and the substrate temperature on the structure of Mo-N coatings produced by arc PVD was investigated. High speed steel substrates were coated at 1.9, 1.5, 1.2, 0.8 and 0.4 Pa nitrogen pressures by using -150, -250 and -350 V bias voltages. The coating procedures were performed at two temperature ranges: at 4 105 10 degreesC and at 300-380 degreesC. At the nitrogen pressure of 1.9 Pa and bias voltage of -150 V, delta-MoN (hexagonal) phase was obtained. When the pressure decreased to 1.5 and 1.2 Pa, mixed phases of delta-MoN and gamma-Mo2N were produced. At lower nitrogen pressures of 0.4 and 0.8 Pa coatings consisted mainly of gamma-Mo2N. At higher bias voltages it was impossible to produce single phase delta-MoN even at the highest nitrogen pressure (1.9 Pa). At lower nitrogen pressure and higher bias voltages cubic gamma-Mo2N was the predominant phase. Lower substrate temperatures promoted the formation of delta-MoN phase. The hardness of the coatings changed from 3372 to 5085 kg/mm(2) depending on the ratio of phase components in the coatings. (C) 2002 Elsevier Science B.V. All rights reserved.