This study was aimed to explore the relationship between humic acid (HA, as a model NOM) adsorption on and stabilizing multiwalled carbon nanotubes (MWNTs) in water with a focus on the effects of HA, MWNT and solution properties. It was found that MWNT-surface area-normalized adsorption of HAs (Q(SA)) increased with increasing outer-diameter of the MWNTs and decreasing polarity of the HAs. However, at low pH values (ca. <4) or high ionic strengths (ca. >1 mmol L-1 Ca2+), the HA adsorption decreased with decreasing polarity of the HAs. The MWNT stabilization increased with increasing Q(SA), but the increase leveled off when Q(SA) exceeded a threshold value markedly lower than the maximum Q(SA), especially for the MWNTs with relative large outer-diameters. On the whole, the Q(SA)-normalized MWNT stabilization, presenting the capability of the MWNT-adsorbed HAs for the MWNT stabilization, increased with increasing HA polarity and solution pH, but with decreasing Ca2+ concentration. However, the stabilized MWNTs by the HAs with greater polarity could be more subject to destabilization by Ca2+. The results of this study are believed to shed light on predictive understanding the interaction between MWNTs and NOM and the environmental behavior of MWNTs. (C) 2012 Elsevier B.V. All rights reserved.