In this paper, US nanocrystallites are prepared by encapsulation in poly (ethylene oxide) polymer (PEO). It is found that PEO could provide a confined environment for particle neucleation and crystallite growth of US nanocrystallites. The absorption onset of the US nanocrystallites shifts to shorter wavelength with decreasing the amounts of Cd2+ in PEO, indicating the particle size could be controlled under the change of the Cd2+ /PEO relative concentration. Their corresponding photoluminescence peaks are all located at about 630 nm, which origins from the sulfur vacancies, but the intensities increase with increasing the molar ratio of Cd2+ /PEO. When US nanocrystallites are modified by excessive S2-, the emissions from band gap are observed and the peak positions show coherent shift with the absorption onsets. Such a blue-shift of the emissions could be explained by size quantization effects. (C) 2003 Elsevier Science B.V. All rights reserved.