The majority of carboxylic acid groups formed after treatment of a single-walled carbon nanotube (SWCNT) material with nitric acid are present on carboxylated carbonaceous fragments (CCFs) (see figure). These CCFs are removed from the SWCNTs by treatment with NaOH solution and identified as carriers of the COOH functionality, implying that previous reports claiming side-wall functionalization of the SWCNTs with COOH groups should be reconsidered.