Block copolymers with and without reactive functionalities can improve fracture resistance in brittle epoxies even when added in relatively small amounts (<5 wt %). At certain compositions, amphiphilic block copolymers spontaneously self-assemble into vesicles, spherical micelles, or wormlike micelles in thermoset resins, and these morphologies are retained with the full curing of the resins. The addition of such block copolymers leaves the glass-transition temperature of these blends relatively unchanged, whereas the fracture resistance increases up to a factor of 3.5 for the vesicle-modified blends. For epoxies modified with block copolymers self-assembled into a spherical geometry (vesicles or spherical micelles), the fracture resistance scales with the ratio of the interparticle distance to the average vesicle (or spherical micelle) diameter (D-i/D-P) and increases as this quantity is reduced. Greater adhesion between the vesicle and epoxy resin improves the fracture resistance only at higher values of D-i/D-P, at which the materials are more brittle. Debonding and subsequent matrix plastic deformation are identified as the toughening mechanisms in these blends. (C) 2003 Wiley Periodicals, Inc.