The first water-soluble perester that can serve as a surface-active photoinitiator for polymerization has been synthesized. This photoinitiator showed a good solubility in water and a sufficient molar absorptivity in the WA region. While heat-activated decomposition of this peroxide in an aqueous solution leads mainly to heterolytic cleavage of the dioxy bond, its photochemical decomposition gives predominately homolytic cleavage. When the perester was used as an initiator in microemulsion polymerization of styrene, stable and relative monodispersed polystyrene latices were Obtained. The produced polystyrene has a high molecular weight in the range of 10(4)-10(6) and a narrow molecular weight distribution (MWD = M-W/M-n similar to 2.5). The initiator is located at the interface, and the polymerization may be initiated with the separation of two radicals which are generated at the interface. The latex characteristics such as particle size, polydispersity, molecular weight, and molecular weight distribution depend on the nature of the surfactant and the way by which the radicals are generated.