The evolution of single-wall carbon nanotubes (SWNT) under hydrothermal treatment was investigated. Single-wall carbon nanotubes were used as the starting material and treated at temperatures between 200 and 800 degreesC and pressure of 100 MPa. The microstructural evolution was studied by means of transmission electron microscopy and Raman spectroscopy. After treatment at 800 degreesC, the single-wall nanotubes transformed completely to shorter multiwall carbon nanotubes and graphitic particles. The mechanism by which the observed evolution occurs is discussed.