Supercritical water (SCW) in the presence and absence of oxygen is used for the first time for the opening and thinning of multiwall carbon nanotubes (MWNTs). The influence of variation of pressure, temperature, and time on the opening and thinning of MWNTs is examined. In SCW, opening and thinning of MWNTs is observed both in the presence and absence of oxygen. In addition, the presence of oxygen (similar to2 mmol) shows improved thinning of MWNTs with the collapsed outer graphene layers tending towards the inner layers. The morphology of MWNTs are critically analyzed using transmission electron micrographs (TEM) and Raman spectra. (C) 2002 Elsevier Science B.V. All rights reserved.