The large amount of terminal hydroxyl groups of amphiphilic multiarm star copolymers with hyperbranched polyethylenimine (PEI) as the hydrophilic core and poly(epsilon-caprolactone) (PCL) arms as the hydrophobic shell were completely transformed into the radical-crosslinkable methacrylate (MA) groups. The resulting PEI-b-PCL-MA polymers could self-assemble into vesicles in water, which was verified by dynamic light scattering (DLS) and transmission electron microscopy (TEM). After crosslinking the intravesicular MA groups, covalently stabilized vesicles (CSVs) were generated. These CSVs were further characterized by DLS and TEM, and it was found that the corona of the vesicles was not the simple double-layer structure, but contained a certain amount of PEI-b-PCL unimolecular micellar units between the double-layer. These CSVs could accommodate both apolar and polar guests using their hydrophobic PCL zones and void cores, respectively. Moreover, these CSVs showed superior capacities for apolar guests to their noncrosslinked precursors and the corresponding traditional amphiphilic multishell star polymers. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 227-236, 2012