The pyrolysis of perhydropolysilazane in anhydrous ammonia has been studied up to 1000 degrees C through the analysis of the gas phase and the characterization of the solid residue by 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 three main steps : (1) below 400 degrees C, evaporation of residual solvent; (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; (3) from 600-1000 degrees C, completion of the formation of an amorphous hydrogenated solid with composition close to silicon nitride.