B-C-N thin films of a wide composition range were deposited by reactive DC magnetron sputtering of targets with different B/C ratios in an Ar/N-2 atmosphere. The bonding characteristics of these amorphous films were investigated by Fourier-transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results of both characterisation methods indicate that real ternary compounds in which all three elements are bonded to each other are only formed when at least one element has a low concentration in the film-and therefore could be considered as an impurity. Otherwise the deposited material tends to a phase separation into binary compounds and single phases. (C) 2004 Elsevier B.V. All rights reserved.