The synthesis of pH-sensitive star polymers with 100% hyperbranched cores and polyacrylic or polymethacrylic acid arms is reported. A series of stars obtained via atom transfer polymerization of tert-butyl acrylate and methacrylate onto poly(arylene oxindole)s were subjected to acidic hydrolysis to obtain structures with polyacid arms. The obtained polyacid stars were characterized by NMR and FTIR spectroscopy. Dynamic and static light scattering experiments were performed to provide information regarding the shape of these macromolecules in solution. In an aqueous solution, the stars formed aggregates with a radius of gyration reaching 130 nm and aggregation numbers of up to 280 stars per particle. Similar to other polyacid-based copolymers with different branched topologies, the dimensions and aggregation numbers were found to decrease with increasing pH. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5074-5086, 2011