We study the influence of the parameters of modifying oxygen-containing atmosphere (the degree of rarefaction, temperature and time of modification) on the oxynitriding of titanium alloys. It is shown that, as the degree of rarefaction of the atmosphere increases and the temperature of modification decreases, the process of phase formation on the surface of titanium alloys evolves in the direction of formation of oxynitrides, whereas the period of modification does not affect the structural and phase state. As the oxygen content decreases, the composition of titanium oxynitride approaches the equiatomic composition. This guarantees the increase in surface microhardness and the improvement of the corrosion resistance of titanium alloys in oxygen-free acids.