In this final part of our investigation, the copolymers prepared (reported in Part III) have been pyrolysed to yield silicon oxynitride. Thermogravimetric studies performed on several copolymers in nitrogen, ammonia/nitrogen mixtures, air and oxygen are discussed in the present paper. One copolymer was used for further investigation and its pyrolysis conditions (variation of pyrolysis atmosphere according to temperature) were optimized to produce the maxim um yield of white amorphous residue wh ich was crystallized by heating to 1600 degrees C to produce silicon oxynitride powder. The powder particles were equiaxed in shape and in the size range 50-150 nm but in agglomerates of size 0.3-0.1 mu m. Crystallization at higher temperatures resulted in a mixture of a silicon nitride-silicon oxynitride.