The stability of single-wall carbon nanotubes under hydrothermal conditions (100 MPa pressure, from 30 min to 48 h in the temperature range from 200 to 800 °C) has been investigated. The resultant products were characterized by Raman spectroscopy, x-ray diffraction, and transmission electron microscopy. The stability range of single-wall carbon nanotubes (SWCNTs) under hydrothermal conditions suggests that they, similar to fullerenes, can only survive mild and short-term treatment in high-temperature, high-pressure water. SWCNTs gradually transform into multiwall carbon nanotubes (MWCNTs) and polyhedral graphitic nanoparticules. After 48 h at 750 °C only the Raman spectra characteristic of graphitic carbon were observed. Transmission electron microscopy revealed that after 800 °C and 48 h of treatment SWCNTs fully transformed into MWCNTs and polyhedral carbon nanoparticles.