4-Nitrophenol (4-NP)or 4-nitrobenzyl alcohol (4-NBA) are used as voltammetric and X-ray photoelectron spectroscopic (XPS) labels for carboxylic acid groups on the surface of single-walled carbon nanotubes, “bamboo-like” and “hollow-tube” multi-walled carbon nanotubes. The surface carboxyl groups are first converted to the corresponding acyl chlorides, and coupled to the labels via the formation of an ester linkage. The voltammetric reduction of the arylnitro label allows the number of carboxyl groups on the surface to be estimated. Alternatively the percentage of the elemental surface composition corresponding to the carboxyl groups can be measured using XPS via the emission from the N1s level of the label. The number of electroactive quinonyl groups present on the surface relative to the number of carboxyl groups can also be determined voltammetrically by measuring the area under the voltammetric wave corresponding to the surface quinone groups. By combining these voltammetric and XPS results the relative percentages of the O1s spectral peak, which correspond to carboxyl and quinonyl groups, can then be determined, which also allows, by difference, an estimate of the percentage elemental surface composition of total other oxygen-containing functionalities present such as lactones, ethers, aliphatic ketones etc. The effect of standard acid pre-treatments used to introduce carboxyl groups onto the CNT surface (such as stirring in relatively concentrated mixtures of nitric and sulfuric acids) on the number of each type of oxygen functionality is compared. In all cases the number of carboxyl groups is found to increase, but the number of quinonyl and other oxygen-containing functionalities is also found to increase, often to a greater extent than the carboxyl groups.