The structural and chemical stability of multiwall carbon nanotubes (MWNTs) in ceramic nanocomposites prepared by spark plasma sintering was studied. High resolution electron microscopy, X-ray diffraction and Raman spectroscopy were used to evaluate any degradation of the MWNTs. They were found to be well preserved in alumina after sintering up to 1900°C/100 MPa/3 min. In boron carbide, structural degradation of MWNTs started from 1600°C when sintered for 20 min. Multiwall carbon nanotubes maintained their high aspect ratio and fibrous nature even after being sintered in boron carbide at 2000°C for 20 min. However, no Raman vibrations of MWNTs were observed for nanocomposites processed at temperatures 2000°C, which indicates that they were severely degraded. Structural preservation of MWNTs in ceramic nanocomposites depends on the ceramic matrix, sintering temperature and dwell time. Multiwall carbon nanotubes were not preserved for matrices that require high sintering temperatures (>1600°C) and longer processing times (>13 min).