The use of oil-soluble initiators in free-radical miniemulsion polymerization has increased due to their ability to generate radicals primarily within monomer droplets. Existing theories concerning the nucleation mechanism for oil-soluble initiators suggest that a single radical must be formed within a particle for propagation to occur, despite the fact that an oil-soluble initiator molecule decomposes to form two radicals. According to existing theories, the primary source of nucleating radicals may be either within the particle or from the small amount of initiator in the aqueous phase. At the nanometer size of miniemulsion particles, concentration variables do not adequately describe the reactions in a particle, and use of such variables may lead to the inaccurate description of mechanisms within a particle. Application of the kinetic Monte Carlo algorithm allows for the simulation of individual reactive events, such as propagation or termination, while tracking macroscopic observables, such as conversion and molecular weight. Comparison of the simulation results to experimental data for styrene miniemulsion polymerization indicates that desorption of radicals may be more significant at smaller particle sizes and lower reaction temperatures. Oil-soluble initiator radicals generated in the aqueous phase are found to be insignificant under typical conditions in the miniemulsion polymerization of styrene. (C) 2008 Wiley Periodicals, Inc.