A series of well-defined hybrid block copolymers PMACyPOSS-b-PMMA and PMAiBuPOSS-b-PMMA exhibiting high POSS weight contents have been synthesized by RAFT polymerization and further studied as modifiers for epoxy thermosets based on diglycidyl ether of bisphenol A. The hybrid block copolymers self-assembled within the epoxy precursors into micelles possessing an inorganic core and a PMMA corona. Thanks to the presence of the PMMA blocks that remain miscible until the end of the reaction, curing of the resulting blends afforded nanostructured hybrid organic/inorganic networks with well-dispersed inorganic-rich nanodomains with diameters on the order of 20 nm. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4343-4352, 2011