Hydrogenated amorphous carbon layers (a‐C:H) formed by plasma deposition on silicon and germanium substrates have been depth profiled by 5‐keV Ar+sputtering and analyzed with Auger and photoelectron spectroscopy, secondary ion and secondary neutral mass spectroscopy. Reference data have been established for quantitative analysis, compound identification and the exclusion of the experimental artifact of ion‐bombardment induced carbide formation. The composition, binding state and some structural parameters ofa‐C:H layers on Si and Ge substrates have been investigated for various deposition parameters. In each case, a carbidic phase is found in the interface to the substrate. The adhesion of the layers depends mainly on the overlap of this interfacial carbide with the adjacent regions, whereas the hardness of the layers can be characterized by the structure and composition of the layer itself. Changes caused by tempering of the layers are also discussed.