Novel quaternary Si-B-C-N materials are becoming increasingly attractive because of their possible high-temperature and harsh-environment applications. In the present work, amorphous Si-B-C-N films were deposited on Si and SiC substrates by reactive dc magnetron cosputtering using a single C-Si -B or B4C-Si target in nitrogen-argon gas mixtures. A fixed 75% Si fraction in the target erosion areas, a rf induced negative substrate bias voltage of -100 V, a substrate temperature of 350 degrees C, and a total pressure of 0.5 Pa were used in the depositions. The corresponding discharge and deposition characteristics (such as the ion-to-film-forming particle flux ratio, ion energy per deposited atom, and deposition rate) are presented to understand complex relationships between process parameters and film characteristics. Films deposited under optimized conditions (B4C-Si target, 50% N-2+50% Ar gas mixture), possessing a composition (in at. %) Si32-34B9-10C2-4N49-51 with a low (less than 5 at. %) total content of hydrogen and oxygen, exhibited extremely high oxidation resistance in air at elevated temperatures (even above 1500 degrees C). Formation of protective surface layers (mainly composed of Si and O) was proved by high-resolution transmission electron microscopy, Rutherford backscattering spectrometry, and x-ray diffraction measurements after oxidization. Amorphous structure of the Si-B-C-N films was maintained under the oxidized surface layers after annealing in air up to 1700 degrees C (a limit imposed by thermogravimetric analysis in oxidative atmospheres). (c) 2008 American Vacuum Society.