Boron-carbon-nitrogen (B-C-N) thin films were synthesized using an electron beam excited plasma (EBEP)-CVD method and their properties and structure investigated. By controlling the flow rate ratios of process gases, we obtained films with composition expressed as BxCyN, where x = 0.9-4.7 and y = 0.5-6.0. Hydrogen content is very small, of the order of 1 at. %. The hardness of the films increases monotonically with increasing boron content and reaches 29 GPa. Results of X-ray photoelectron spectroscopy, Fourier transform infrared transmission and X-ray diffraction measurements show that while the films consist of a sp(2)-bonded B-C-N structure in a boron poor region, a sp(3)-bonded structure exists as well as the sp(2)-bonded structure in a boron rich region. (C) 2002 Elsevier Science B.V. All rights reserved.