Binary composites formed by individually mixing exfoliated graphene oxide modified with amine terminated poly(butadiene acrylonitrile) (GA) and a spherical micelle forming poly(ethylene oxide)-b-poly(ethylene-alt-propylene) (OP) diblock copolymer with a thermoset epoxy, and the associated GA/OP/epoxy ternary composites, were prepared and studied as a function of the molecular weight between cross-links. The rigid GA filler dispersed well in the cured epoxies as established by transmission electron microscopy (TEM). The toughening efficacy of GA alone was found to depend strongly on the modifier concentration and the matrix cross-link density with an optimal 1.7-fold increase in the critical strain energy release rate (G(Ic)) over the neat epoxy obtained With a 0.04 wt % loading, in the most lightly cross-linked (M-c = 6100 g/mol) material. Addition of 5 wt % OP to this epoxy resin enhanced G(Ic) by a factor of 12, Combining the hard GA and soft OP modifiers at the same loading levels (0.04 and 5 wt %, respectively) resulted in 18 times the G(Ic) of the unmodified material, a 31% improvement over the effect anticipated by simple addition of the fracture properties of the binary composites. Decreasing M-c to 700 g/mol eliminated this synergistic effect while reducing the overall improvement in G(Ic) to just 3 times that of the neat epoxy. Topological features on the fracture surfaces, imaged using a scanning electron microscope (SEM), suggest that the Synergistic toughening of the GA/OP/epoxy ternary composite involves concurrent mechanisms operating on different length scales, including micelle cavitation and graphene debonding, resulting in simultaneous shear yielding, crack pinning, and crack deflection.