In this work a single Langmuir probe has been used to determine in-situ the plasma electronic temperature (Te) and the electronic density (ηe) of an rf magnetron sputtering system used to grow CNx films. Sodium chloride substrates and a carbon target (99.999%) in a mixture of Ar/N2 were used. From the voltage–current characteristics, the electronic temperature in the range of 3 to 11 eV and an electronic density in the range of 109 cm–3, depending on the pressure and the relationship Ar/N2 in the gas mixture, were determined. These results were correlated with Fourier Transformed Infrared Transmission Spectroscopy (FTIR) of the films. The stress bands in the infrared spectra were found located at 1357, 1589 and 2247 cm–1; these bands are related to the primary (C–N), double (C=N) and triple (C≡N) bonds, respectively. On the other hand, the chemical reactions in a discharge are generated basically by electronic impacts; so, knowing the temperature and electronic density, the plasma processes can be controlled during the film preparation to favor CNx deposition.