The chemistry of cyclosilazanes and borazines is a topic of current interest in view of preceramic polymers which yield on pyrolysis various useful ceramic materials, e.g. silicon and boron carbide, silicon and boron nitride, and ternary or quaternary mixtures of these materials. These materials are hard and have high oxidative and thermal stability. Other useful properties are resistance to corrosion, thermal shock and creep, low electrical conductivity and low coefficient of thermal expansion. One of the best application prospects is the use of the preceramic polymers as protective coating material for carbon fibres.