The microstructure and composition of boron nitride:carbon (BN:C) thin films prepared by reactive r.f. diode sputtering of a B4C target in mixed Ar and N-2 discharges were determined by high-resolution electron microscopy, electron diffraction, infrared spectroscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy. Films were prepared with three characteristic phase compositions; cubic BN:C (c-BN), turbostratic BN:C (t-BN), and phase mixture of c-BN and t-BN on Si(001) substrates. While keeping the B/N ratio close to unity, phase structures were mainly correlated to the energy and flux of impinging (Ar+ + N-2(+)) ions towards the negatively d.c. biased substrate. At a constant ion flux, substrate biases of 500 V yielded c-BN films while biases lower than 300 V resulted in t-BN. Films prepared with the same ion flux and with biases between 300 and 500 V consisted of c-BN and t-BN phase mixtures. The film phase evolution in c-BN films was from an initial amorphous BN:C (a-BN) layer, over a highly oriented t-BN layer with the c axis parallel to the film surface, to a c-BN layer exhibiting (110) preferred orientation. Films consisting of c-BN and t-BN phase mixtures were non-textured nano- to sub-microcrystalline. The c-BN layers/grains showed twinning on the c-BN(111) lattice planes. As-deposited films contained as much as 5-15 at.% of C with mainly C-C and B-C bonds. The film C content decreased with increasing volume fraction of c-BN.