Boron nitride films fabricated by ion beam assisted deposition (IBAD) have been analyzed using polarized infrared reflection spectroscopy (PIRR), generalized variable angle of incidence spectroscopic ellipsometry (gVASE), elastic recoil detection analysis (ERDA), and in situ ellipsometry during plasma etching. Hexagonal boron nitride (h-BN) films, with properties very similar to the interface material on top of which cubic boron nitride (c-BN) nucleates, have been prepared. PIRR and gVASE results show that these films are biaxially anisotropic. The h-BN basal planes are aligned to the surface normal and exhibit a preferential alignment parallel to the direction of the boron vapor stream. This behavior is unrelated to the substrate crystal orientation. The observed preferential alignment of the h-BN basal planes cannot be explained by compressive stress or ion beam induced processes. The biaxially anisotropic optical properties of these h-BN films can be employed to fabricate special optical thin film devices but require the application of generalized ellipsometry also for in situ measurements.