A new kind of polyborosilazane was synthesized from hexamethylcyclotrisilazane by exposure to boron trichloride. The pyrolysis of the precursor was carried out at 1250, 1600, and 1800degreesC in nitrogen for a dwelling time of I h. Characterizations were performed with Fourier transform infrared, X-ray photoelectron spectroscopy, thermogravimetric analysis, and XRD techniques, and the results showed that the boron content of the precursor increased as the exposure time increased, whereas the C/Si ratio remained constant. Minor amounts of Si and C remained in residue pyrolyzed at 1250degreesC, but they were completely removed when their residue was further sintered at 1800degreesC. At that point, it only consisted of hexagonal BN crystals. Therefore, the polymer could be used as a precursor to BN in some special applications. (C) 2004 Wiley Periodicals, Inc.