A commercial multiwall carbon nanotube and its carboxylated derivate (CNTC and COOHC, respectively) was used after purification to study the competitive adsorption of phenol (P) and m-chlorophenol (CP) from 0.1 M aqueous NaCl solutions without external pH control. The adsorption takes place practically exclusively on the external surface of the nanotubes. The uptake of P is suppressed in comparison to its single solute behaviour on both nanotubes, independently of the initial pollutant concentration. The uptake of CP however is more sensitive to the concentration and the surface chemistry of the nanotube. The measured co-adsorption isotherms were compared to the isotherms calculated from the competitive Langmuir model (CLM). Preferential adsorption of CP was observed in about 95% of the relative concentration range. The total adsorption may exceed the corresponding single component sorption capacity. (c) 2012 Elsevier Inc. All rights reserved.