Single walled carbon nanotubes (SWNTs) are commonly purified by competitive oxidation using mineral acids, which generates molecular debris not removed from the sample by conventional water washing. Washing with dilute aqueous base at room temperature removes much of this debris, producing samples with increased purity; the base converts acidic groups to their conjugate salt, increasing the solubility of the partially oxidised organic debris in water. Removal of this debris is crucial if direct functionalisation of the SWNTs is required. This method is shown to be applicable to a range of SWNT samples. (C) 2008 Elsevier B.V. All rights reserved.