The stability of C-60 fullerene particles in water affects its mobility, bioavailability, and toxicity to organisms. Natural organic matters (NOMs) have pronounced effects on the aggregation behavior of C-60 fullerene. This study was to examine the effects of NOM structural properties on the aggregation behavior of fullerene water suspension (FWS). Fulvic acid (FA), tannic acid (TA), and two structurally different humic acids (HA1 and HA7) were studied. HA1 and HA7 were sequentially extracted HAs, where HA7 was more hydrophobic than HA1 and had a higher molecular weight. Aggregation was induced by addition of varying amounts of Ca2+ to the FWS with 2 mg/L of each NOM. The absolute value of zeta potential vertical bar zeta vertical bar of pure FINS increased after addition of any type of NOM. Addition of Ca2+ to the FWS + NOM system decreased vertical bar zeta vertical bar of fullerene almost uniformly for all types of NOM. FWS critical coagulation concentration (CCC) was equal to 14.5, 6.5, 5.4, and 3.7 mM Ca2+ for HA7, HA1, FA, and TA, respectively. The order of increasing CCCs was positively correlated to the NOMs molecular weight and negatively to their polarity. A nearly constant zeta for FWS + NOM system at a wide range of Ca2+ concentrations suggested the steric stability rather than electrostatic one. This study highlighted the role of NOM in the fate of manufactured nanoparticles in the environment and linked the structural properties of NOM to their interaction with manufactured nanoparticles. (C) 2012 Elsevier Inc. All rights reserved.