A water-insoluble dye was used as a probe to study the particle nucleation mechanisms in the semibatch emulsion polymerization of methyl methacrylate. The experimental results show that homogeneous nucleation plays a key role in the particle formation period. When the surfactant concentration ([S]) is above its critical micelle concentration (CMC), the mixed modes of particle nucleation (micellar and homogeneous nucleation) are operative in the polymerization system. In the absence of micelles, homogeneous nucleation is the predominant mechanism. Limited flocculation of latex particles becomes more important in controlling the particle concentration as the initiator concentration ([I]) is increased from 2.66 to 31.92 mM. When [S] is kept constant at 12 mM, the degree of formation of particle nuclei in the aqueous phase increases with increasing [I]. Monomer droplet nucleation however, cannot be ruled out. This is because diffusion of dye molecules from monomer droplets to the growing latex particles originating from homogeneous nucleation is prohibited and in the absence of micelles an appreciable amount of dye still can be detected in the resultant latex particles. (C) 2000 Elsevier Science Ltd. All rights reserved.