Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si2N2O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures.α‐Si3N4is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si2N2O is approximately 19 GPa, comparable with the hardness of Si3N4hot isostatically pressed with 2.5 wt% Y2O3, while the fracture toughness level is around 3.1 MPa. m1/2, being approximately 0.8 MPa.m1/2lower. The three‐point bending strength increased with HIP temperature from approximately 300 t