Semiconductor CdS nanoparticles were synthesized by the sonochemical reduction of a mixed aqueous solution ofCdCl2, Na2S2O3 and (CH3)2CHOH in Ar atmosphere at room temperature. The results of a detailed investigationwith X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectrum and optical absorption spec-troscopy are reported. A clearly red shift of the absorption edge and a broad absorption band related to the surfacestates of nanoparticles have been observed in the optical absorption spectra with the increasing of CdS particle size.The mechanisms of the CdS nanoparticle formation and size growth during sonochemical irradiation were discussed.This convenient method is found to be an efficient way to produce other chalcogenides as well.