CNx-thin films have been deposited by plasma-activated chemical vapour deposition with capacitively or inductively coupled r.f. plasma. Acetylene, methane, carbon monoxide and tetracyanoethylene have been used as carbon precursor. A strong dependence of the layer properties on the precursor was found. In some films the nitrogen to carbon ratio was close to that of C3N4. The highest nitrogen content was observed in films made from carbon monoxide as precursor in an inductively coupled argon/nitrogen plasma. The nitrogen was mainly incorporated with covalent nitrogen-carbon single bonds. X-ray diffraction measurements showed no reflections indicating crystallinity. In small grains (length similar to 10 mu m) found on the layer surface the stoichiometry corresponded nearly to that of C3N4, the oxygen content is very low. Further characterizations by TEM are intended.