A simple, inexpensive, and reproducible procedure is described for large-scale synthesis of highly stable nanocrystalline ZnS powders. Cysteine-capped ZnS nanocrystals (NCs) were produced by a colloidal aqueous synthesis, employing a ligand-competition mechanism in which sulfide was introduced into a preformed zinc-cysteine solution. The synthesis procedure resulted in highly concentrated ZnS NC solutions (similar to 100 mM) which could be ethanol-precipitated, redissolved, and dried to produce fine powders stable for more than 30 months at 4 degrees C. The NC powders were readily dissolved in aqueous solvents to concentrations as high as 300 mM. ZnS NCs could be prepared without cysteine capping, but only at extremely dilute concentrations (similar to 0.2 mM ZnSO4) as per Sooklal et al. J. Phys. Chem. 100, 4551 (1996). The 30-month-old ZnS NC powders retained their original optical and photocatalytic properties and could be handled much like routine shelf chemicals, unaffected by ambient air or moderate moisture and temperature. UV/vis absorption spectroscopy showed band gap energies (E-g) ranging from 4.82 eV (257 nm lambda(max)) to 4.47 eV (277 nm lambda(max)) for ZnS samples prepared with 0.25-2.0 initial sulfide ratios las compared to zinc). Samples stored at 4 degrees C for 30 months showed equivalent band gap energies and spectral profiles. The average NC particle size was estimated to be 6.08 +/- 0.76 mm by high-resolution transmission electron microscopy. Selected-area electron diffraction and X-ray diffraction analyses concurred in suggesting a hexagonal crystal structure, with diffractions near d = 3.1, 1.9, and 1.6 Angstrom. The average NC composition of size-fractionated samples was estimated to be Cys(1)Zn(7)S(6). p-Nitrophenol, a model organic, was photocatalytically degraded using 30-month-oId ZnS NC powders dissolved in an aqueous buffer. Rates of degradation (first-order rate constant k = 0.261 min(-1); t(1/2) = 2.66 min) were comparable to those of experiments using freshly prepared ZnS NCs (first-order rate constant k = 0.247 min(-1); t(1/2) = 2.80 min), further demonstrating the longterm stability of thus-produced NC powders.