This paper studies the phase separation in poly(ether imide) (PEI) modified epoxy resin using imidazole (C(11)Z-CNS) as epoxy hardener to control its morphology. The sponge-like phase structures were founded at higher PEI concentration (10-25 phr), while homogeneous structures are formed at low PEI concentration (5 phr). The effects of PEI concentration on curing kinetics and phase structures were studied by differential scanning calorimeters (DSC) and scanning electron microscopy (SEM). It is shown that although the addition of PEI does not change the curing mechanism, the separated morphology becomes finer at high PEI concentration. The curing rate and conversion decrease with the increase of the content of PEI. The chain growth polymerization of these systems caused an early gelation (conversion < 10%) and early freezing of morphologies. The evolution of phase separation in the early stage was monitored by synchrotron radiation small angle X-ray scattering (SR-SAXS) and transmission electronic microscopy (TEM). It is suggested that the formation of sponge-like phase structure could be attributed to the strong viscoelastic effects in the early stage of phase separation. (C) 2003 Elsevier Ltd. All rights reserved.