A series of indecomposable additives was tested for its effect on the nitridation of fly ash. The carbonitridation reaction was carried out in a tubular high-temperature furnace. SiAlON (silicon aluminum oxynitride)-based powders were prepared from fly ash/carbon mixture in a nitrogen atmosphere. Fly ash from the Huntley and Hatfield power plant was examined in this paper. The additives, in the amount of 4 wt %, are represented by several inorganic salts. Systematic analysis made it possible to select proper elements or groups having a strong effect on the nitridation reaction. The results revealed the possibility of manufacturing of expensive SiAlON whiskers at a low cost. This group of additives was never tested in the literature. A nitrogen/oxygen analyzer measured nitrogen content in the final product. Weight losses were also measured to indicate the extent of the evaporation of the solid materials. Scanning electron microscopy and an energy-dispersive X-ray spectrometer were used to characterize the powder of the final product.