PS-b-PCL block copolymer is used to study its influence on the phase evolution of epoxy resin/polyetherimides (PEI) blends cured with methyl tetrahydrophthalic anhydride. The effect of PS-b-PCL on the reaction-induced phase separation of the thermosetting/thermoplastic blends is studied via optical microscopy, scanning electron microscope, and time-resolved light scattering. The results show that secondary phase separation and typical phase inverted morphologies are obtained in the epoxy/PEI blends with addition of PS-b-PCL. It can be attributed to the preferential location of the PS-b-PCL in the epoxy-rich phase, which enhances the viscoelastic effect of epoxy/PEI system and leads to a dynamic asymmetry system between PEI and epoxy. The PS-b-PCL block copolymer plays a critical role on the balance of the diffusion and geometrical growth of epoxy molecules. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1395-1402