A facile, room temperature and environmentally benign green route for the production of monodispersed nanocrystalline ZnS in 7 min by ultrasonic irradiation in aqueous solution of room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium ethyl sulfate, ([EMIMI][EtSO4]), is proposed. The X-ray diffraction (XRD) studies display that the products are well crystallized in the form of hexagonal structure. Energy dispersive X-ray spectroscopy (EDX) investigations reveal that the products are extremely pure. The morphology of as-prepared nanoparticles was characterized by scanning electron microscopy (SEM). The absorption spectra of the product reveal band gap energy of about 4.13 eV, which shows a blue shift of 0.59 eV and can be attributed to the quantum-confinement effect of the ZnS nanoparticles. Based on the experimental results, a possible formation mechanism of the monodispersed nanocrystalline ZnS in the ultrasonic-assisted synthesis in aqueous solution of the RTIL is presented. (c) 2008 Elsevier B.V. All rights reserved.