The growth mechanism of CdS ultrafine particles in water-in-oil microemulsions was investigated. The growth occurs via a irreversible particle coagulation process. Its rate constant is dependent on the agglomeration number of a particle n. At small agglomeration numbers, the rate constant k is about 10(7)dm(3)mol(-1)s(-1). It decreases dramatically to about 10(3)dm(3)mol(-1)s(-1) at a critical agglomeration number n(c) followed by a further decrease in k above 2n(c). This causes sharpening of the size distribution near n(c) and 2n(c). A simple model based on the coagulation process with n-dependent rate constants is proposed, which explains well the experimental results. The effects of initial mixing and the feed ratio of Cd2+ to S2- are understood well by the proposed model. The relationship between n(c) and the stables supramolecules composed of CdS particles was discussed.