The effect of background gas environment on the purity of reactively-deposited nitride films has been studied. Especially, the relation between the oxygen background pressure and its incorporation into the TiN film is investigated in this study. We have developed a UHV sputtering system and deposited TiN films under two different base pressure conditions: one was a UHV condition less than 10 -6 Pa, and the other was 1 ×10 -4 Pa of oxygen. The oxygen content of the films was examined with X-ray photoelectron spectroscopy. While no trace of oxygen was detected in the TiN film deposited under the UHV condition, 10 at.% of oxygen was observed in that deposited with the O 2 introduction. The extent of O 2 incorporation into the TiN film is discussed based on the difference in sticking characteristics of oxygen and nitrogen on the titanium surface.