The present article considers instability phenomena during the sputtering of a single metallic target of titanium in a reactive atmosphere involving argon+oxygen+nitrogen gases. Since the process using two reactive gases is much more complex than one reactive gas, the transition zone of the reactive mode must be defined taking into account oxygen and nitrogen mass how rates. A two-dimensional representation delimiting boundaries of the instability region and depending on both mass flows is proposed for radio frequency and direct current (dc) polarization of the titanium target. This diagram provides operating conditions favorable to deposit oxy-nitride coatings with a modulated oxygen or nitrogen composition. It is also shown that the supply of one reactive gas affects consumption and behaviors of the other gas as well as its own characteristics. A critical region is then defined in which the process is trapped in reactive mode and can not go back to elemental conditions by changing the mass flow rate of only one reactive gas. Kinetics and pollution of the titanium surface are discussed from measurements of de potential for various operating points. It is shown that comportments of oxygen and nitrogen towards metallic or poisoned target are interdependent and a surprising evolution of the time of pollution against oxygen and nitrogen introduction is observed. These original results are supported by optical emission spectroscopy experiments. Emission intensities of nitrogen and oxygen species can also be used to monitor the state of pollution of the titanium target and so to control instabilities of the two reactive gases sputtering process.