Formation of cubic boron nitride by r.f. magnetron sputtering has been studied with O-2 addition to the common working gas Ar/N-2. The chemical and the phase composition were determined with Auger electron spectroscopy sputter depth profiling and Fourier transform infrared spectroscopy. The result shows that oxygen hinders the formation of cBN in sufficient nitrogen-supply, but facilitates the growth of cBN in insufficient nitrogen-supply. With insufficient nitrogen-supply, there exists an optimal oxygen-supply in the working gas that promoted the establishment of the stoichiometric condition in the growing film. O-concentration in the film increases with oxygen-supply in the working gas. cBN forms only when the oxygen concentration is below 5% and c(N)/c(B) (ratio of concentration of nitrogen atoms and boron atoms) is 1 in the film. (C) 2009 Elsevier B. V. All rights reserved.