Abstract Boron nitride thin films were prepared by plasma-enhanced chemical vapour deposition (PECVD) of BF 3, H 2 and N 2 gas mixtures. Fourier transform IR (FTIR) spectroscopy and X-ray analysis reveal that the films deposited on crystalline substrates show signs of cubic and hexagonal phases present within the atomic network. The index of refraction and the film thickness were determined by ellipsometry. The deposition rate increases with temperature and saturates at high plasma powers. Values of n are in the range 1.63–1.77, and the optical gap varies from 5.0 to 5.6 eV depending on the preparation conditions (the presence of contaminants also alters this property). The ratio of N 2 to H 2 in the flow affects the properties of the films.