High density silicon oxynitride ceramics were fabricated by reaction-sintering a mixtureof silicon nitride and silica without the use of sintering aids. Precursor silicon nitride compacts were prepared by conventional means after which they were subjected to a lowtemperature oxidation heattreatment (∼1000ºC) producing a composite silicon nitride/silica compact. Oxidized compacts were then reaction-sintered in a nitrogen atmosphere at temperatures between 1400 and 1800ºC using a range of protective ‘powder-bed’ compositions. A ‘powder-bed’ comprising a mixture of boron nitride, silicon nitride and silica was found to be most effective in preventing decomposition and subsequent weight loss. Nearly complete reactive transformation to silicon oxynitride was observed under optimised sintering conditions.