A water-insoluble dye was used to probe the particle nucleation loci in the semibatch emulsion polymerization of styrene (ST). Most of the dye molecules are present in the monomer droplets and some of them are solubilized in the monomer-swollen micelles. The extremely hydrophobic dye molecules cannot diffuse from the monomer droplets and micelles to the growing latex particles. Thus, determination of the amount of dye incorporated into the resultant latex particles provides valuable information on the particle nucleation mechanism. The mixed modes of particle nucleation (micellar and homogeneous nucleation) were proposed when the surfactant concentration ([S]) is above its critical micelle concentration (CMC). In the absence of micelles (i.e., [S] < CMC) most of the latex particles are produced via homogeneous nucleation. Monomer droplet nucleation cannot be ruled out because an appreciable amount of dye still can be detected in the resultant latex particles. The influence of the initiator concentration on the particle nucleation mechanisms was also investigated. The experimental data, again, support the proposed mixed modes of particle nucleation when [S] > CMC. Homogeneous nucleation plays an important role in the particle formation period when [S] < CMC.