ZnS nanoparticles are synthesized by gamma-irradiation of aqueous solution containing Zn2+ and thiol (RSH). Monodispersed ultra-small ZnS particles with 1.5 nm diameter, characterized by a band edge at 238 nm are produced at low doses. At higher doses, larger particles absorbing at lambda > 260 nm are obtained. When excited at 250 nm, they exhibit an emission band at 400 nm. The early steps of formation and coalescence of ZnS particles are investigated by pulse radiolysis. (c) 2006 Elsevier B.V. All rights reserved.