We report a comparative study on gas and liquid phase oxidative treatments of SWNTs, using a HiPCO produced raw material with a broad distribution of nanotube diameters. The effects of these two processes were correlated using absorption and Raman spectroscopy (radial breathing and G modes) and TEM. We conclude that liquid phase oxidation is a continuous diameter-selective process eliminating narrower SWNTs through oxidizing sidewalls of SWNTs. Gas phase oxidation is a more discrete process requiring high activation energy for diameter-selective purification. Unlike liquid phase oxidation the gas phase process preferentially oxidizes SWNTs without introducing sidewall defects. (C) 2003 Elsevier B.V. All rights reserved.