A series of well-defined poly(ethylene oxide) (PEO)-based star copolymers with controlled number of arms, compactness, and compositions were prepared by atom transfer radical polymerization using a simple "arm-first" method. Tuning the composition of the arms allowed these star polymers to stabilize either oil-in-water or water-in-oil emulsions. Stable oil-in-water emulsions were obtained with star copolymers containing only hydrophilic PEO arms due to their better affinity to the aqueous phase. In contrast, the insertion of a fraction of hydrophobic poly(butyl acrylate) arms into the star structure significantly changed the surfactant behavior of the star copolymers, leading to the formation of stable water-in-oil emulsions. Additionally, in both cases, an extremely low surfactant concentration, <0.01 wt % vs total weight of water and xylene used for emulsion formation, was sufficient to generate stable emulsions.