The effect of natural organic matter (NOM) on Cu(II) adsorption by multi-walled carbon nanotubes (MWCNTs) was investigated. Particular attention was paid to the relationship between NOM properties and NOM adsorption and their effects on the Cu(II) adsorption by MWCNTs. The results showed that the NOM adsorption data were well fit to the Freundlich isotherm model, and the MWCNTs that had higher oxygen content had lower K-F (Freundlich constants) values. The K-F values for the different types of NOM varied greatly and were proportional to the aromatic carbon content, the specific ultraviolet absorbance at 275 nm, and the TOC-normalized fluorescence intensity of the F3 peak at similar to 460 nm. In contrast, they had negative linear correlations with the absorbance ratios at 250 nm and 365 nm and the TOC-normalized fluorescence intensity of the F1 peak from 250 nm to 300 nm. The results indicated that pi-pi and hydrophobic interactions were the predominant NOM adsorption mechanisms by the MWCNTs. The Cu(II) adsorption by the MWCNTs increased with increasing NOM concentrations due to the complexing of Cu(II) with the adsorbed NOM. This was confirmed by XPS and FTIR spectra. The increased degree of Cu(II) adsorption was positively correlated with the carboxyl carbon content, carboxyl group content, and the polarity index of the NOM. The findings in this study highlight that the NOM characteristics are responsible for controlling the NOM adsorption and its effects on heavy metal sorption by MWCNTs. (C) 2012 Elsevier By. All rights reserved.