The effects of various reaction parameters on the styrene miniemulsion polymerizations stabilized by nonylphenol polyethoxylate with an average of 40 ethylene oxide units per molecule (NP-40) and dodecyl methacrylate (DMA) at 80 degrees C were investigated. These parameters include the concentrations of DMA ([DMA]), NP-40 ([NP-30]), sodium persulfate ([SPS]), and 2,2＇-azobisisobutyronitrile ([AIBN]). A water-insoluble dye was also incorporated into the reaction system to gain a better understanding of the related particle nucleation mechanisms. The polymerization rate decreases with increasing [DMA], whereas it increases with increasing [NP-40], [SPS] and [AIBN]. Several competitive events (e.g, coalescence among the monomer droplets, nucleation in the monomer droplets and micelles, formation of particle nuclei in water, and particle growth) occur simultaneously in the course of polymerization. This is due to the fact that the steric stabilization effect provided by NP-40 is greatly reduced at 80 degrees C and the CMC of the miniemulsion is far below the NP-30 concentrations used in this work. Mixed modes of particle nucleation are operative in this reaction system, but monomer droplet nucleation becomes more important by increasing [DMA] or decreasing [NP-40], [SPS] and [AIBN].