The sterically stabilized emulsion polymerization of styrene initiated by a water-soluble initiator at different temperatures has been investigated. The rate of polymerization (R-p) versus conversion curve shows the two non-stationary-rate intervals typical for the polymerization proceeding under non-stationary-state conditions. The shape of the R-p versus conversion curve results from two opposite effects-the increased number of particles and the decreased monomer concentration at reaction loci as the polymerization advances. At elevated temperatures the monomer emulsion equilibrates to a two-phase or three-phase system. The upper phase is transparent (monomer), and the lower one is blue colored, typical for microemulsion. After stirring such a multiphase system and initiation of polymerization, the initial coarse polymer emulsion was formed. The average size of monomer/polymer particles strongly decreased up to about 40% conversion and then leveled off. The initial large particles are assumed to be highly monomer-swollen particles formed by the heteroagglomeration of unstable polymer particles and monomer droplets. The size of the "highly monomer" swollen particles continuously decreases with conversion, and they merge with the growing particles at about 40-50% conversion. The monomer droplets and/or large highly monomer-swollen polymer particles also serve as a reservoir of monomer and emulsifier. The continuous release of nonionic (hydrophobic) emulsifier from the monomer phase increases the colloidal stability of primary particles and the number of polymer particles, that is, the particle nucleation is shifted to the higher conversion region. Variations of the square and cube of the mean droplet radius with aging time indicate that neither the coalescence nor the Ostwald ripening is the main driving force for the droplet instability. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 804-820, 2003.