By co-ammonolysis of MeHSiCl(2) and Me(2)SiCl(2) and following thermal polymerization, polysilazanes as the precursors to Si3N4/SiC ceramic fibre were synthesized. The tendency to cross-link in thermal polymerization is suppressed as the result of the introduction of Me(2)SiCl(2) as a starting material. The reactions occurring during thermal polymerization of ammonolysis products are discussed based on infrared (i.r.) and H-1 nuclear magnetic resonance (NMR) spectra analysis. The pyrolysis process of polysilazanes is examined in nitrogen and in an NH3 atmosphere. It is shown that the formation of SiC results from the pyrolysis of Si-CH3 groups in polysilazanes by means of the formation of Si-CH2-Si intermediate bonds. The structure and properties of ceramics derived from the pyrolysis of polysilazane and polycarbosilane in N-2 or NH3 atmosphere are compared and discussed. Si-N-C,ceramic, which was proved to be a composite of alpha-Si3N4 and beta-SiC, exhibits better thermal stability at temperatures higher than 1300 degrees C.