Amphiphilic block copolymers are receiving increasing attention because of their rich lyotropic and thermotro- pic phase behavior which can be utilized for emulsifica- tion, pharmaceutical applications, and the synthesis of advanced materials.1,2 In particular, block copolymers of the Pluronic type3 (PEO-PPO-PEO block copoly- mers) have been studied intensely. However, because of their inherent polydispersity and specific properties of different batch-polymerization charges, a quantitative interpretation of experimental data with theoretical models is often difficult. Thus, there has arisen a need for water-soluble model amphiphilic block copolymers with narrow molecular weight distributions, controllable block lengths, and high purity. These requirements are met by polystyrene-poly(ethylene oxide) (PS-PEO), polybutadiene-poly(ethylene oxide) (PB-PEO), and polyisoprene-poly(ethylene oxide) (PI-PEO) block co- polymers, which can be synthesized by living anionic polymerization. Of these block copolymers, PB-PEO and PI-PEO are particularly suitable for experimental investigations of aqueous phase behavior, because they are directly soluble in water even at large hydrophobic block lengths. This is in contrast to PS-PEO, where the high glass-transition temperature of the PS block mostly prevents direct dissolution in water