This paper deals with the successful preparation of cubic boron nitride (c-BN) films by use of an electrically conducting boron carbide (B4C) target in reactive r.f. sputtering. It describes the influences of the most important sputter parameters. It turned out that the B:N ratio of the sputtered layers was stabilized in the order of 1 by using Ar-N-2 gas mixtures with >6% N-2. The carbon content in the films could be reduced to <5 at.% by varying the Ar-N-2 gas mixture. With respect to c-BN formation important parameters have been found in the ratio of target to substrate power and the negative substrate bias voltage, Almost single-phase c-BN films have been deposited at substrate temperatures <300 degrees C. The films were characterized by electron probe microanalysis, IR spectroscopy, high resolution transmission electron microscopy and transmission electron diffraction. The films revealed a nanocrystalline microstructure 10-40 nm in size. Hardness as determined by nanoindentation measurements was as high as 60 GPa.