X-sialon has been produced by hot pressing Si3N4-Al2O3-SiO2 powder mixtures and Si3N4 powder-mullite gel mixtures at 1650 degrees C. The formation mechanism of X-sialon has been studied and is correlated with the processing technique for the two preparation routes. Microprobe analysis of the obtained X-sialon phase, combined with previous observed compositions and literature formulae suggests that, at 1650 degrees C, X-sialon exists as a narrow solid solubility region on the Si3N4-mullite line in the Si3N4-SiO2-Al2O3-AlN phase diagram. The physical, mechanical and chemical properties of X-sialon have been evaluated. X-sialon has a modest hardness of 1280 kg mm(-2), a fracture toughness of 1.7 MPa m(1/2), and an elastic modulus of 213 GPa. X-sialon exhibits excel lent chemical stability in contact with iron-based alloys at 1200 degrees C.