In this study the formation of approximately spherical assemblies of nanocrystalline primary particles of CdS in a CS2-water-ethylenediamine (CWE) microemulsion induced by ultrasonic irradiation is described. CS2 was employed as the sulfur source for CdS and also as the oil phase in the microemulsion. The particles were studied with X-ray powder diffraction, transmission electron microscopy, UV/Vis absorption spectroscopy, and photoluminescence spectroscopy. A blue shift compared to bulk CdS was observed in absorption and photoluminescence spectra,