ADVANCED ceramic materials that can withstand high temperatures (over 1,500 degrees C) without degradation or oxidation are needed for applications such as structural parts for motor engines, gas turbines, catalytic heat exchangers and combustion systems(1,2). Hard, oxidation-resistant ceramic composites and coatings are also in demand for use on aircraft and spacecraft, Silicon nitride (Si3N4) and silicon nitride/carbide (Si3S4/SiC) composites are good candidates for such high-temperature applications(2,3). Commercial Si3N4 parts can be used in oxidizing environments up to 1,200-,1,300 degrees C (ref. 4), but are oxidized at still higher temperatures. Here ne describe the synthesis of a covalent ceramic composite which is resistant to oxidation at temperatures up to 1,600 degrees C. The composite is formed from an amorphous silicon carbonitride which crystallizes at high temperature into a composite of alpha-Si3N4 microcrystals and alpha-SiC nanocrystals. The oxidation resistance stems from the formation of a passivating surface layer of SiO2 a few micrometres thick.