Synergic effects of poly(vinyl butyral) (PVB) on rubber toughening of epoxies have been examined in detail using two different liquid rubbers. The PVB is seen to be miscible with the epoxy used. Incorporation of the PVB into liquid rubber/epoxy blends lowers the rubber particle size from 0.76 mu m to 28 nm, leading to a tremendous increase in fracture toughness, represented by the critical strain energy release rate, G(Ic), from 100 J/m(2) for the neat epoxy to 2015 J/m(2) for the ternary blend of PVB/liquid rubber/epoxy (14/14/72 by weight). Such an enormous improvement in toughness is largely ascribed to the unique nanostructure of the rubber phase and enhanced deformability of the epoxy. Rubber particle cavitation and shear yielding of the matrix are found to be the main toughening mechanisms. This study provides a novel approach to achieving nano-dispersion of a typical liquid rubber in toughening epoxies which conventional routes fail to deliver. Such an approach serves as first direct evidence about that nano-sized rubber particles other than block copolymers considerably toughen epoxies by over 20 fold. (C) 2015 Elsevier Ltd. All rights reserved.