The mechanisms of microemulsion polymerizations stabilized by sodium dodecyl sulfate in combination with pentanol were investigated with a water-insoluble dye as the probe. The major parameters chosen for study were the types of initiators [water-soluble sodium persulfate (SPS) vs oil-soluble 2,2'-azobisisobutyronitrile (AIBN)] and the polarity of the monomers [relatively hydrophobic styrene (ST) vs relatively hydrophilic methyl methacrylate (MMA)]. Both continuous particle nucleation and limited particle flocculation had a significant influence on the polymerization kinetics. For the polymerizations investigated in this work, the relatively low initiation efficiency of AIBN resulted in a reaction system showing a quite different particle nucleation mechanism than that of the ST polymerization with SPS. The formation of particle nuclei in water was suppressed to some extent, and microemulsion droplet nucleation predominated in the ST polymerization initiated by AIBN. Homogeneous nucleation played an important role, and a mixed mode of particle nucleation (microemulsion droplet nucleation and homogeneous nucleation) was operative in the NMA polymerization. The MMA polymerization experienced stronger particle flocculation than its ST counterpart. (C) 2005 Wiley Periodicals, Inc.