The high‐temperature chemical stability of hot‐pressed Si3N4was studied between 600° and 1450°C. Reactions were followed by X‐ray diffraction and scanning electron microscopy. In air, this material begins to oxidize at 700° to 750°C; a distinct amorphous siO2surface layer results after 24 h at 750°C‐Concomitant formation of cristobalite occurs, depending on exposure time, and is enhanced as temperature is Increased. Magnesium and calcium magnesium silicates form above 1000°C. The data suggest that impurities, e.g. Mg, Ca, and Fe, greatly lower the oxidation resistance o