A systematic analysis is presented on the influence of parameters of synthesis on the formation and optical properties of US nanocrystals prepared using dimethyl formamide as solvent and stabilizing agent. The parameters considered are the molar ratios of cadmium to sulfur, initial concentration of Cd (2+) and S2- at a fixed molar ratio and atmospheric conditions. Optical absorption and photoluminescence measurements were carried out during the synthesis to monitor the changes in optical properties. The results clearly indicate that the nanocrystals size could be tuned by varying the molar ratio of the starting materials. Ambient conditions are found to play a vital role on the stability and optical properties of the nanocrystals whereas the concentration of the starting materials had little influence on the optical absorption and luminescence properties. These results are expected to help in developing a better understanding of the mechanisms of solution growth and size stabilization of semiconductor nanocrystals during chemical synthesis. (C) 2006 Elsevier Ltd. All rights reserved.