The effects of a small amount of carboxylic monomers (AA and MAA) on the styrene miniemulsion polymerizations stabilized by SDS/DMA or SDS/SMA are studied. A mixed mode of particle nucleation (monomer droplet nucleation and homogeneous nucleation) is operative during polymerization. Formation of particle nuclei in the aqueous phase predominates in the SDS/SMA stabilized polymerization in the absence of carboxylic monomers. The relatively stable monomer droplets exhibiting an extremely large droplet surface area become the primary particle nucleation loci when a small amount of AA or MAA is used as the comonomer. On the other hand, homogeneous nucleation plays a crucial role in the polymerizations stabilized by SDS/DMA. The experiment using the less hydrophilic MAA as the comonomer shows an intermediate behavior in the polymerization kinetics. A mechanistic model is proposed for the enhanced monomer droplet nucleation in the polymerizations containing carboxylic monomers. (C) 2000 Elsevier Science Ltd. All rights reserved.