Magnetron sputtering discharges for the deposition of cubic boron nitride have been investigated using optical emission spectroscopy and Langmuir double probes. Spatially resolved measurements revealed the inhomogeneous discharge structure with respect to the distribution of excited species, electron density and electron temperature. The optical emission spectra were analysed by means of the corona model. The vibrational excitation of the nitrogen molecule was examined. The ratio of selected Ar and N 2 emission intensities provided a qualitative measure of the electron temperature, which was determined by probe measurements. However, the intensity ratios of Ar and Ar + lines did not correspond to changes in electron temperature. Varying the electron density in a wide range, it could be demonstrated that the excitation mechanisms do not remain constant, mainly due to multistep excitation. The vibrational temperature of N 2 (C 3 IIΠ u ) has been found to be much higher in the case of r.f. sputtering than in the d.c. mode.