ZnS and other metal-sulfide nanoparticles are known to persist as intermediates during precipitation and dissolution of their respective mineral phases. However, the mechanisms that enable these nanoparticles to persist in surface waters and sediment porewaters are unknown. Humic substances and other hydrophilic organics can stabilize ZnS nanoparticles by adsorbing to particle surfaces and preventing aggregation. In this study, the stability of aqueous ZnS nanoparticles was investigated by assessing the role of low-molecular weight organic ligands (oxalate, serine, cysteine, and mercaptoacetate) during precipitation of ZnS. Zn(NO₃)₂ and Na₂S were dissolved  (2 uM each) in laboratory aqueous solutions buffered at pH 7.5 and containing organic ligands. Particle formation and size was monitored over time by dynamic light scattering. Zn speciation was measured by anodic stripping voltammetry to confirm that Zn was coordinated to sulfide during the aggregation experiments and was not in the form of dissolved Zn-organic complexes. The ZnS aggregate growth rates varied by orders of magnitude, depending on the type and concentration of organic ligand in solution. Growth rates were slowest in the presence of thiol-containing ligands, cysteine and mercaptoacetate. In contrast, ZnS aggregation rates were generally not affected by oxalate and serine, which contain carboxylate and amine functional groups. Thermodynamic stability constants for Zn-thiol complexes are greater that those for the other Zn-organic complexes. Thus, slow aggregation of ZnS nanoparticles may be caused by specific attachment of the thiol on surface Zn sites. These results indicate that thiol-containing organics are important for stabilizing metal-sulfide nanoparticles in the aquatic environment.