X-ray diffraction studies of boron nitride powders synthesized from boric oxide and ammonia at 900 degrees C indicates a turbostratic structure which is characterized by the absence of ordering in the third dimension. Turbostratic boron nitride (t-BN) powders are found to be unstable and decompose into ammonium berate compound (NH4 . B5O8 . 4H(2)O) after long storing periods. Thermal transformation of t-BN into the stable, hexagonal ordered layer lattice structure was promoted with heat treatments at 1500-1800 degrees C for 15 min-5 h. The transformation and degree of crystallization was observed with Xray diffraction analysis. The effect of crystallization period and temperature on the particle sizes and shapes of the powders was examined with transmission electron microscope. BET surface area measurements of the powders were also performed for the characterization of powder properties. (C) 1997 Elsevier Science Limited.