Some experiments on the growth of cubic boron nitride (c-BN) show that the content of c-BN in BN films depends on the film thickness. Non-cubic phases, such as amorphous and hexagonal BN, can be identified, particularly near the substrate. To understand these phase variations, elemental depth profiles of thin BN films were measured by high resolution elastic recoil detection (ERD). BN films have been prepared by r.f. glow discharge (r.