Fine physicochemical characterization has allowed proposing of a mechanism for the nitridation pathway of a polyborosiloxane polymer into a new ceramic material in the SiBON system. A polyborosiloxane, a polymer consisting of Si O B linkages, was synthesized by the condensation reaction between tetrachlorosilane SiCl 4 and boric acid B(OH) 3 . The polymer was then thermally nitridated under flowing ammonia into an oxynitride of boron and silicon. This conversion was observed using various structural techniques: chemical analysis, X-ray diffraction, infrared spectroscopy and X-ray photoelectron spectroscopy. The nitridation process can be divided in two main stages: (i) between 400 and 800°C, B N bonds are formed by B O bond cleavage; (ii) above 1000°C, Si N bonds are formed by Si O bond cleavage. The oxynitride remains amorphous even at 1300°C. Pyrolysis up to 1700°C led to a partial crystallization of hexagonal boron nitride.