The main purpose of this work is the preparation of single layer films of molybdenum oxynitride, MoNxOy, The films were deposited on steel substrates by dc reactive magnetron sputtering. The depositions were carried out from a pure Mo target and varying the flow rate of reactive gases. This allowed tuning of the crystallographic structure between insulating oxides and metallic nitrides and consequently changes in the electronic, mechanical and optical properties of the material. X-ray diffraction (XRD) results revealed the presence of molybdenum nitride for the films with low oxygen fraction: face-centered cubic phases (gamma-MO2N) for low nitrogen flow rate or cubic MoNx and hexagonal phase (delta-MoN) for high nitrogen flow rate. The increase of oxygen content induces an amorphization of the nitride phases and the appearance of MoO3 phases. The increase of the oxygen fraction in the films induces also a high decrease in the film's hardness. Residual stresses were compressive, in the range of very few tenths of GPa to -2 GPa. These results will be presented as a function of the deposition parameters, the chemical composition and the structure of the films. (c) 2005 Elsevier B.V. All rights reserved.