Chemical modi. cation of MWCNTs via oxidation proved to be a useful tool to improve the suspension stability and solubility of MWCNTs in aqueous solution. Multi-walled carbon nanotubes (MWCNTs) were oxidized using different oxidizing agents and the produced oxidized MWCNTs were characterized using different techniques. IR measurements showed the presence of carboxylic acid function groups especially for the MWCNTs oxidized with nitric acid and hydrogen peroxide. Oxidation of MWCNTs increased their solubility in aqueous solution and hence enhances the contact between the carbon nanotubes and the water and pentachlorophenol molecules. The oxidized MWCNTs were used to study the removal of pentachlorophenol (PCP) from aqueous solutions. The equilibrium adsorption of PCP on the oxidized MWCNTs at various temperatures was studied and the adsorption equilibrium was well described using different adsorption models. The thermodynamic of adsorption was studied at different temperatures and the results showed that the adsorption process was product favored, and becomes more so at higher temperature, since the adsorption is endothermic in general. (C) 2008 Elsevier B. V. All rights reserved.