Amphiphilic star copolymers were synthesized by sequential monomer and cross-linker additions using group transfer polymerization (GTP). Benzyl methacrylate (BzMA) and methoxy hexa(ethylene glycol) methacrylate (HEGMA) served as the hydrophobic and hydrophilic monomers, respectively, whereas the also hydrophobic ethylene glycol dimethacrylate (EGDMA) was used as the cross-linker. In total, twelve star copolymers were prepared, covering three different overall hydrophobic compositions, 39, 53 and 70% w/w, and four different architectures, AB star-block, BA star-block, heteroarm star and random star. The theoretical molecular weight of each arm was kept constant at 5000 g mol(-1). The molecular weights and molecular weight distributions of the linear precursors and of all the star copolymers were characterized by gel permeation chromatography (GPC) in tetrahydrofuran (THF), while their compositions were confirmed by proton nuclear magnetic resonance (H-1 NMR) spectroscopy. Moreover, all the star copolymers were characterized by static light scattering (SLS) in THF to determine the absolute weight-average molecular weight, M-w, and the weight-average number of arms. After polymer characterization, xylene-water and diazinon (pesticide)-water emulsions were prepared using these star copolymers as stabilizers at 1% w/w copolymer concentration and at different overall organic phase/water ratios. The most important factor in determining the emulsion type was the star copolymer composition in hydrophobic units. The four most hydrophilic star copolymers (39% w/w hydrophobic composition) always formed o/w emulsions, while the four most hydrophobic star copolymers (70% w/w hydrophobic composition) always formed w/o emulsions. The type of the emulsion in the case of the star copolymers with the more balanced composition, 53% w/w hydrophobic units, also depended on the emulsion content in the organic solvent, similar to particulate-stabilized emulsions. Considering that the best o/w emulsifier is that star copolymer which can emulsify the largest quantity of organic phase in water resulting in low viscosity, o/w emulsions without excess oil or water phase, it appeared that the most hydrophilic random copolymer star is the optimal emulsifier. Moreover, this star copolymer presented the smallest droplet size in its emulsions. It is also noteworthy that the resulting emulsions almost never had high viscosity, a feature attributable to the compact nature of star polymers. (c) 2005 Elsevier Ltd. All rights reserved.