This work investigates the adsorption of four groups of organic compounds (OCs): (1) nitroaromatics, (2) nonpolar aliphatics, (3) monoaromatics and (4) polycyclic aromatic hydrocarbons (PAHs) on multiwalled carbon nanotubes (MWCNTs). In order to investigate the influence which oxygen containing functional groups exert on the adsorption process, three MWCNTs were used: the pristine (original, as-received) MWCNTs (OMWCNT) and two MWCNTs functionally modified by acid treatment of OMWCNT during 3 h and 6 h (FMWCNT3h, FMWCNT6h). All adsorption isotherms well fitted with the Freundlich model. The nonlinearity of the isotherms ranged from 0.418 to 0.897. The results show that K-d values for PAHs increased with increasing specific surface areas (SSAs). The adsorption affinities of the larger molecular size OCs (PAHs) were higher than those of the smaller size OCs (benzene, toluene and hexane) which is probably due to their large contact area with the surface of the adsorbent. Adsorption of OCs on MWCNTs was mainly controlled by hydrophobic interactions, except for the nitroaromatic compound, as shown by the increasing adsorption affinities with the compound's hydrophobicity. K-ow-normalized adsorption coefficients (K-d/K-ow) for all the investigated compounds on all the MWCNTs followed the order: nonpolar aliphatic < monoaromatics < PAHs < nitroaromatic, implying that pi-pi interactions enhanced the adsorption of aromatics on the MWCNTs. It can be concluded that the strong adsorptive interactions between the MWCNTs and nitroaromatics was due to the pi-pi electron-donor-acceptor (EDA) interaction between nitroaromatic molecules (electron acceptors) and the highly polarisable graphene sheets (electron donors) of the carbon nanotubes. (C) 2013 Elsevier B.V. All rights reserved.