The pyrolysis of polyborosilazane in anhydrous ammonia has been studied up to 1200 degrees C through the analysis of the gas phase and the characterization of the solid residue by elemental analyses, thermogravimetric analysis, Fourier transform-infrared analysis, X-ray photoelectron spectroscopy, X-ray diffraction and Si-29 cross-polarization/magic angle spinning-nuclear magnetic resonance. The pyrolysis mechanism involves four main steps : (1) below 400 degrees C, an evaporation of residual solvent and reaction with ammonia leading to an increase of nitrogen content; (2) from 400-600 degrees C, reaction with ammonia leading to an increase of nitrogen content and formation of preceramic polymer-ceramics intermediate solid with a three-dimensional network along with evolution of gaseous species; (3) from 600-800 degrees C, completion of loss of C-H functionalities and progress of formation of an amorphous ceramic, accompanied by evolution of CH4 and H-2; (4) from 800-1200 degrees C, completion of formation of an amorphous hydrogenated Si-B-O-N solid composed of SiN4-xOx(x=0,1,2,3) and BN3-yOy (y=0, 1), along with evolution of H-2