A new X-ray spectrometric method for the control of plasma deposition was developed in the laboratory. While this method is devoted primarily to d.c. reactive sputter deposition, it is also useful for hybrid PECVD deposition of carbon. The intensity of the X-ray emission induced in the film is correlated with the thickness of the film and the type of substrate. The electron backscattering of the substrate is significant for low film thicknesses. The so-called "ionization function" was measured down to 1 keV and was in good agreement with higher energy measurements. The fine structure of the C Kalpha emission band changes with the deposition conditions. It is shown that the films have an amorphous carbon structure. These X-ray results are confirmed by Raman spectroscopy.