Nonaqueous microemulsions containing formamide, the anionic surfactant AOT (bis(2-ethylhexyl)sulfosuccinate sodium salt), sodium bromide (NaBr), and either of the monomers hexyl methacrylate or styrene are polymerized at 60°C. For both monomers, the final product is a stable, bluish dispersion of particles of ca. 80 nm diameter. Based on phase behavior studies as a function of NaBr concentration, we describe how a systematic variation of composition and monomer causes changes in reaction rates and latex characteristics. The monomer solubility in the continuous phase plays an important role in determining the final polymer characteristics. Decreasing monomer solubility shifts the mechanism from one similar to solution polymerization to one closer to traditional emulsion polymerization.