The batch emulsion polymerization kinetics of styrene initiated by a watersoluble peroxodisulfate at different temperatures in the presence of sodium dodecyl sulfate was investigated. The curves of the polymerization rate versus conversion show two distinct nonstationary-rate intervals and a shoulder occurring at a high conversion, whereas the stationary-rate interval is very short. The nonstationary-state polymerization is discussed in terms of the long-term particle-nucleation period, the additional formation of radicals by thermal initiation, the depressed monomer-droplet degradation, the elimination of charged radicals through aqueous-phase termination, the relatively narrow particle-size distribution and constant polydispersity index throughout the reaction, and a mixed mode of continuous particle nucleation. The maximum rate of polymerization (or the number of polymer particles nucleated) is proportional to the rate of initiation to the 0.27 power, which indicates lower nucleation efficiency as compared to classical emulsion polymerization. The low activation energy of polymerization is attributed to the small barrier for the entering radicals. The overall activation energy was controlled by the initiation and propagation steps. The high ratio of the absorption rate of radicals by latex particles to the formation rate of radicals in water can be attributed to the efficient entry of uncharged radicals and the additional formation of radicals by thermally induced initiation.