Exploiting the high brilliance of synchrotron radiation, we performed surface-sensitive and depth-resolved x-ray scattering experiments on thin films of boron nitride grown on Si(001) substrates. In-plane strains of different structural phases, namely turbostratic and cubic, grain sizes and textures were determined. Annealing the films up to temperatures of1000 °Cleads to large strain relaxation of about 70&percent;, while the grain size stays constant at 80 Å. ©1998 American Institute of Physics.