Given the wide presence of heterogeneous natural organic matter (NOM) and metal ions (Naþ/Ca2þ/Mg2þ), as well as their significant role in governing nanoparticle stability in aqueous environments, it is of great importance to understand how the molecular weight (MW)-dependent physicochemical properties of NOM impact fundamental transportation processes like the aggregation of engineered nanoparticles (ENPs) in the presence of Naþ/Ca2þ/Mg2þ. Here, we report on the aggregation behavior of a model ENP, fullerene nanoparticles (nC60) in the presence of five MW fractions of Suwannee River NOM (Mf-SRNOMs, separated by ultrafiltration techniques) and three electrolytes (NaCl, CaCl2 and MgCl2). We found that in all NaCl treatments and low concentration CaCl2/MgCl2 treatments, the enhancement of nC60 stability positively correlated with the MW of Mf-SRNOMs. Whereas, the stability efficiency of identical Mf-SRNOM in different electrolytes followed an order of NaCl > MgCl2 > CaCl2, and the enhanced attachment of nC60-SRNOM associations was observed in high MW Mf-SRNOM (SRNOM>100 kD and SRNOM 30e100 kD) at high concentration CaCl2/MgCl2. Our results indicate that although the high MW NOM with large humic-like material is the key component for stabilizing nC60 in monovalent electrolyte, it could play a reversed role in promoting the attachment of nC60, especially in long term aggregations and at high concentrations of divalent cations. Therefore, a detailed understanding of the effects of heterogeneous NOM on the aggregation of ENPs should be highly valued, and properly assessed against different cation species and concentrations.