The regularities formation of oxynitride layers on titanium with diffusion treatment has been investigated. Commercially pure titanium, alpha-, pseudo-alpha-(Ti-Al-V-Mo-Zr) and (alpha+beta)-(Ti-Al-V) alloys were used. Oxynitrides were formed by the modifying of titanium nitride with oxygen according to the following scheme: TiN(x) + O(2) -> TiN(x)O(1-x). The oxynitriding process was carried out by introducing the controlled oxygen-containing medium into the system at finish stage of nitriding (during cooling). The influence of nitriding parameters (temperature, nitriding time, nitrogen partial pressure) and oxynitriding parameters (temperature, oxynitriding time, oxygen partial pressure) on phase composition as well as mechanical and corrosion properties of the oxynitride coatings were studied. It was determined that the intensity of oxynitriding is more efficient when TiN(x) is near its lower limit of homogeneity and increases with lowering of oxygen partial pressure in range 0.0001...0.01 Pa and temperature of oxynitriding in range 1223...923 K. The corrosion properties of oxynitride coatings were improved at higher nitrogen contents in the oxynitride compounds. (C) 2011 Elsevier B.V. All rights reserved.