Crack-free and highly smooth boron-rich boron nitride (BN) films with a thickness of up to 2 mum have been prepared on Si and SiO2/Si substrates by vacuum or inert atmosphere pyrolysis at 900-1100 degreesC of single spin-coated polyborazine films. The thickness of the BN films depends on the polymerization time of the borazine monomer and the polymer concentration, and the surface morphology on pyrolytic conditions. As the pyrolytic temperatures increase, the films displayed lower IR absorption for residual N-H bonds and higher crystallinity with better preferential orientation along the substrate surface, but severe inter-diffusion phenomena at the interface. Microstructural development of the prepared films was thoroughly characterized by SEM, XRD, SPM, IR, SIMS and high resolution TEM.