Metal-sulfide (ZnS and HgS) colloids are found in a variety of aquatic systems including mine drainage, anaerobic sediment porewater, municipal wastewater effluent, and hydrothermal vent ecosystems. While occurrence and source information have become available, relatively little is known about the processes that enable the persistence of these metal-sulfides as colloids or nanoparticles. The objective of this study was to identify surface interactions that occur between metal sulfide colloids and dissolved natural organic matter (NOM). We hypothesized that stabilization of ZnS and HgS particles by humic compounds during precipitation depends on specific ligand binding on the metal-sulfide surface as well as bulk structural properties of NOM (e.g., aromaticity, molecular weight, hydrophobicity, reduced sulfur content). Precipitation experiments were conducted with simple, low-molecular weight organic acids that are surrogates for naturally-occurring NOM. Particle size was measured using time-resolved dynamic light scattering. Observed growth rates of ZnS and HgS aggregates varied by orders of magnitude, depending on the type and concentration of organic ligand in solution. Thiol-containing ligands were more effective than hydroxyl-containing ligands for decreasing the growth rates of ZnS and HgS particles. The particles were characterized for elemental composition by X-ray photoelectron spectroscopy. The result of an approximate 1:1 metal to sulfur molar ratio suggested that ZnS and HgS were precipitating in the mixtures. We conducted additional metal-sulfide precipitation experiments with aquatic humic substances isolated from several surface water sources. These humic isolates represent a wide range of chemical properties (e.g., molecular weight, aromaticity, reduced sulfur content). Our preliminary findings indicated that observed growth rates of ZnS and HgS particles decreased by at least one to two orders of magnitude when 0.5-1 mg/L peat humic acid was present. Overall, this study suggests the importance of NOM when evaluating the stability of naturally-occurring and anthropogenic metal-sulfide colloids in natural aquatic systems.