The influences of nitric acid oxidation on the surface properties and the adsorption capacity of single-walled carbon nanotubes (SWCNTs) were investigated in this work. To eliminate the size effects on the adsorption capacity, o-xylene and p-xylene were used as model adsorbates. It was found that purification of the SWCNTs by nitric acid significantly increased the internal surface area as well as the micropore volume of the SWCNTs, and introduced oxygen-containing surface groups. The adsorption capacities of the SWCNTs for o-xylene and p-xylene were mainly influenced by the positions of the methyl groups on the xylene molecules and the presence of oxygen-containing groups on the surface of the SWCNTs. Results also indicated that purification greatly changes the adsorption of o-xylene by the SWCNTs. This could be attributed to the dispersive attractions and the electrostatic repulsions between o-xylene molecules and the surface of the purified SWCNTs, which are introduced by the oxygen-containing surface groups. When compared to the as-grown and the purified SWCNTs, activated carbon had a greater adsorption capacity because of its large specific surface area and the absence of oxygen-containing surface groups. However, when the adsorption capacity was calculated based on surface area, the as-grown SWCNTs had a greater adsorption capacity than did the activated carbons because the micropore size of the activated carbon is mainly smaller than the size of a xylene molecule.