X-ray diffraction, solid state MAS NMR and isothermal thermogravimetry were used to study the oxidation mechanism and kinetics of pure X-phase and O'-sialon powders prepared by silicothermal synthesis. X-phase sialon (Si12Al18O39N8) begins to oxidise at above 940 degrees C, forming amorphous silica and progressively losing nitrogen from the SiO2N2 groups to form mullite at 1200-1300 degrees C. O'-sialon is more oxidation-resistant; although significant formation of amorphous silica occurs above 940 degrees C, the crystalline products (silica and a small amount of mullite) appear above 1235 degrees C. At 1400-1600 degrees C, oxidation is progressively hindered by the formation of a protective fused silica surface layer containing octahedral Al. The oxidation kinetics can be described by a parabolic rate law, with temperature coefficients of reaction rate (E-a) of 250 and 454 kJ mol(-1) for X-phase and O'-sialon, respectively. The kinetic results are discussed in the context of previous sialon powder oxidation studies.