Thin films of titanium oxynitride were successfully prepared by dc reactive magnetron sputtering using a titanium metallic target, argon, nitrogen and water vapour as reactive gases. The nitrogen partial pressure was kept constant during every deposition whereas that of the water vapour was systematically changed from 0 to 0.1 Pa. These films were made at room temperature (293 K) (set A) and at 673 K (set B). The study consisted in comparing the evolution of deposition parameters like target potential or deposition rate and physical properties of films for each set. Elemental composition measurements obtained by RBS and NRA revealed a reverse and continuous evolution of nitrogen and oxygen contents. Structure and morphology of the films were analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Films deposited at room temperature became amorphous with an increasing supply of water vapour. A higher substrate temperature led to significant changes of the crystallographic structure: from fcc TiN without water vapour to a mixture of anatase + rutile for large amount of water vapour. The optical transmittance spectra of the film/glass substrate system were measured in the visible region as a function of the water vapour pressure. For both sets A and B, optical transmittance was influenced by the water vapour partial pressure. Electrical conductivity measured against temperature was gradually modified from metallic (sigma(300 K) = 1.49 x 10(4) S m(-1)) to semiconducting behaviour (sigma(300) (K) = 2.15 S m(-1)) with an increasing supply of the water vapour partial pressure. Moreover, coatings prepared at room temperature exhibited a surprising maximum of the electrical conductivity for a small amount of water vapour (set A). Such surprising evolution vanished for set B with a restricted range of conductivity from sigma(300 K) = 1.1 x 10(5) to 7 x 10(2) S m(-1). (C) 2003 Elsevier B.V. All rights reserved.