CERAMICS based on silicon nitride and carbide are strong and stable at high temperatures, and are therefore under investigation for the fabrication of motor and turbine parts(1-3). But silicon nitride decomposes at about 1,400 degrees C in vacuum and 1,775 degrees C in 0.1 MPa nitrogen(4,5), limiting the high-temperature range of its technological uses. Here we describe a boron-containing silicon nitride/carbide ceramic that does not degrade at temperatures up to 2,000 degrees C even in nitrogen-free environments. We synthesize the material in a polymer-to-ceramic transformation(6) from a single polymeric polyborosilazane precursor. On heating at 1,000 degrees C in argon we obtain a ceramic with the composition Si3.0B1.0C4.3N2.0. The ceramic begins to convert to a polycrystalline composite of silicon nitride and carbide (with some non-crystalline boron nitride) at 1,700 degrees C, a process that is completed (without substantial change in elemental composition) at 2,000 degrees C.