Amphiphilic block copolymers provide a unique means for toughening epoxy resins because they can self-assemble into different inclusion shapes before epoxy curing. The two examples reported here are spherical micelles and vesicles, which form in blends containing epoxy and symmetric or asymmetric poly(ethylene oxide)-poly-(ethylene-alt-propylene) (PEO-PEP) block copolymer with PEO volume fractions of 0.5 and 0.26, respectively. The vesicles and spherical micelles were characterized by transmission electron microscopy and small-angle X-ray scattering (SAXS), respectively. SAXS data from the spherical micelles were fit to the Percus-Yevick model for a liquid-like packing of spheres with hard-core interactions. Mechanical properties of spherical-micelle-modified and vesicle-modified epoxies in the dilute limit are compared. The glass-transition temperature and Young's (storage) modulus were tested with dynamic mechanical spectroscopy, and compact-tension experiments were performed to determine the critical plane-strain energy release rate for fracture. Vesicles were most effective in improving the epoxy fracture resistance.