The polymerization of styrene has been studied in microemulsions which were stabilized by cetyltrimethylammonium bromide and different water-soluble cosurfactants such as 2-ethoxyethanol, 2-butoxyethanol, 2-(2-ethoxyethoxy)ethanol and 2-(2-butoxyethoxy)ethanol. The activation energy of polymerization and molecular weight of polystyrene were affected by the molar ratio of cosurfactants to surfactant in the dispersed phase. No particular trend was observed for the effect of the cosurfactant on the particle sizes of latexes. A plausible mechanism of radical chain termination for the polymerization is discussed in relation to the interfacial fluidity of a microemulsion particle.