Formation processes of CdS ultrasmall particles from cadmium complexes of 2-mercaptoethanol (RSH) were investigated by means of stopped-flow and NMR spectroscopies. Two different initial processes depending on the amount of RS(-) ions have been found. The presence of S-4-Cd-10(SR)(12) clusters in solution was observed by NMR spectroscopy. Formation of [S4Cd17(SR)(24)](2+) clusters having a 290-nm absorption peak was suggested. The correlation between the absorption peak and the diameter for polynuclear Cd2+-SR(-) complexes is expressed with the finite-depth quantum well model for ultrasmall particles. Electronic interaction between the capping agent and particle framework was suggested on the basis of the differences observed in the absorption wavelengths of the CdS-RSH clusters and the corresponding CdS-thiophenol clusters.