This study presents first investigations on the high-temperature stability and microstructure evolution of SiOC/HfO(2) ceramic nanocomposites. Polymer-derived SiOC/HfO(2) ceramic nanocomposites have been prepared via chemical modification of a commercially available polysilsesquioxane by hafnium tetra (n-butoxide). The modified polysilsesquioxane-based materials were cross-linked and subsequently pyrolyzed at 1100 degrees C in argon atmosphere to obtain SiOC/HfO(2) ceramic nanocomposites. Annealing experiments at temperatures between 1300 degrees and 1600 degrees C were performed and the annealed materials were investigated with respect to chemical composition and microstructure. The ceramic nanocomposites presented here were found to exhibit a remarkably improved thermal stability up to 1600 degrees C in comparison with hafnia-free silicon oxycarbide. Chemical analysis, X-ray diffraction, FTIR, and Raman spectroscopy as well as electron microscopy (SEM, TEM) studies revealed that the excellent thermal stability of the SiOC/HfO(2) nanocomposites is a consequence of the in situ formation of hafnon (HfSiO(4)), which represents a concurrent reaction to the carbothermal decomposition of the SiOC matrix. Thus, by the annealing of SiOC/HfO(2) materials at 1600 degrees C, novel HfSiO(4)/SiC/C ceramic nanocomposites can be generated. The results presented emphasize the potential of these materials for application at high temperatures.