The mechanism of formation of cadmium sulfide and zinc sulfide ultrafine particles in reverse micelles has been studied, using sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/isooctane as a reverse micellar solution. The particle formation process was followed by the change in UV-visible absorption spectra. The effects of reactant concentration and water content on the particle diameter were also investigated. Particle formation by nucleation and growth occurred very rapidly and was complete within 0.02 s, followed by particle coagulation. The particle coagulation rate constant was found to be controlled only by the intermicellar exchange rate, in the initial stage. With increasing particle diameter, the coagulation was dominated by the statistical distribution of particles among the reverse micelles. As the diameter of particle approached that of the water core in reverse micelle, the coagulation became restricted by the micellar size.