Bundled single-walled carbon nanotubes (SWCNTs) were studied as a function of temperature by means of Raman spectroscopy. The Raman modes exhibit negative temperature shift, reversible for the G band and partially irreversible for the radial breathing modes (RBMs). The softening of the latter is larger for larger diameter tubes, resulting in a better separation of the RBMs after high temperature treatment (HTT). The RBM residual softening vs. treatment temperature demonstrates threshold-like behavior. The temperature-induced changes may be associated with an irreversible weakening of the tube-tube (intertube) interaction possibly due to adsorbent removal or destruction of random intertube C-C bonds. (C) 2008 Elsevier B. V. All rights reserved.