The kinetic behavior for the coagulation of ultrafine CdS particles in sodium bis(2-ethylhexyl)-sulfosuccinate (AOT) reverse micelles was studied. The ultimate particle size distribution was measured by transmission electron microscopy. The time-course variation in the mean particle size was followed by UV-visible absorption spectroscopy. The rate of coagulation is a function of the rate of intermicellar exchange for the particles, the statistical distribution of the particles among the micelles, and the probability of coagulation far two particles contained in the same micelle. The probability of coagulation is much reduced by the absorbed AOT layer on the particles as the size of particles approaches that of the micelles. A population balance framework, concerning particle size distribution and the statistical distribution of the particles among the micelles, is proposed. Calculation shows good agreement with the experimental results, except for the case of high water content, at which Ostwald ripening becomes no longer negligible.