Densely packed multiwall carbon nanotubes, with the innermost tube diameter of approximate to 0.6 cm(-1). frequently 9 band at approximate to 1855cm(-1). High temperature heat treatment in anaerobic condition at;: approximate to 1600 degrees C decreases the intensity for the approximate to 1855 cm(-1) band. In addition, after heat treatment at 1550 degrees C, the approximate to 1855 cm(-1) band appeared for the double-wall carbon nanotubes (approximate to 0.6 nm for inner tube diameter), but not for the single-wall carbon nanotubes (approximate to 1.3 mn diameter). We suggest the existence of chain-like carbon materials inside the original nanotube with approximate to 0.6 nm diameter. Moreover, these materials are synthesized at l 550 degrees C but decomposed at approximate to 1600 degrees C. (c) 2005 Elsevier B.V. All rights reserved.