Poly(oxyethylene)-bl~k-poock-poly(oxypropylene~block-poly(oxyethylene) copolymers were applied as nonionic surfactants in styrene emulsion polymerization. The clouding phenomenon of the surfactant in the aqueous phase and the resulting characteristic partition of the block copolymer between aqueous and organic phase result b, two particularities of this system compared with classical emulsion polymerization: i) bimodal particle size distribution, and ii) unusual dependence of reaction rate on temperature.