Abstract: A boron and silicon oxynitride obtained by preparing a polyborosiloxane precursor, conducting nitriding pyrolysis of the precursor to obtain an amorphous boron and silicon oxynitride, and optionally conducting additional pyrolysis to obtain a crystallized boron and silicon oxynitride. The polyborosiloxane precursor is prepared by condensation, in the presence of a catalyst, of (2) a polychlorosilane represented by the formula R3R4SiCl2 in which the substituents R3 and R4 are identical or different and each represents a hydrogen atom, a chlorine atom, a linear or branched alkyl or alkenyl or alkynyl radical having 5 or less carbon atoms, or an aryl or aralkyl radical having 6 to 18 carbon atoms, with (2) an alkyl borate represented by the formula (R5O)3B in which R5 represents a linear or branched alkyl radical having 1 to 5 carbon atoms.