Mineral sulfide colloids and nanoparticles are important for the aquatic fate and transport of toxic metals such as zinc and mercury in anaerobic environments. The persistence of metal sulfides in the colloidal form is likely to depend on surface interactions with dissolved natural organic matter. In this work, we investigated the sorption of cysteine and serine on ZnS and HgS particles and the implications for colloidal stability. These amino acids were used as model compounds for small molecular weight natural organic acids. Cysteine was found to increase colloidal stability by adsorbing to particle surfaces and modifying the surface potential of the particles. In contrast, serine did not adsorb in appreciable amounts and as a consequence, did not appear to alter surface properties and particle attachment efficiencies. The pH of the aqueous phase was another critical component for controlling aggregation kinetics of cysteine-coated ZnS particles. Colloidal stability was promoted at pH values greater than 7 due to deprotonation of functional groups on the mineral surface and sorbed cysteine molecules. These results indicate that specific surface coordination of thiol-containing natural organic acids is important for the colloidal stability of ZnS, HgS, and other metal sulfides in water.