The hardness and indentation fracture toughness, K-IC of sintered X-phase sialon, produced by simultaneous carbothermal reduction and nitriding of kaolinitic clays, are determined. By comparing the values obtained for the sialon with those for a commercially pure mullite, it is suggested that X-phase sialon is a material with potential for similar applications to those in which mullite is currently used. On the basis of the oxygen/nitrogen ratios of the sialon determined by the inert gas fusion method; the analysis of the energy of the dispersed X-rays (EDAX) from thinned samples in the transmission electron microscope (TEM) and the aluminium/silicon ratios, also determined by the TEM EDAX method, a modified compositional formula is proposed for X-phase.