The polymerization of styrene in oil-in-water microemulsions made with the cationic surfactants dodecyltrimethylammonium bromide or chloride is studied as a function of inorganic electrolyte (KBr, KCl, or K2SO4) concentration. The resulting microlatex is stable, but as the electrolyte concentration increases, both the average radius and the polymer molecular weight decrease. The presence of electrolyte slows the polymerization rate and diminishes final conversion as followed by gravimetry, dilatometry, and calorimetry. Both particle radius, determined by quasielastic light scattering, and molecular weight show only limited growth as styrene conversion increases, suggesting continuous nucleation of latex particles and termination by chain transfer to monomer. Small-angle neutron scattering (SANS) of undiluted parent and polymerized microemulsions shows that a unimodal population of swollen micelles evolves into a bimodal population of empty micelles coexisting with large polymer particles. Structural details of the parent and polymerized microemulsions as determined by SANS are used to assess nucleation mechanisms previously proposed for emulsion polymerization.