Homogeneous solutions of syncliotactic polystyrene (sPS) in diglycidylether of bisphenol A (DGEBA), containing 2.5, 5 and 7.5 wt % of thermoplastic with or without 0.5 and 1 wt % of poly(styrene-b-ethylene oxide) (PS-b-PEO) block copolymer, were polymerized using a stoichiometric amount of an aromatic amine hardener, 4,4'-methylene bis (3-chloro-2,6-diethylaniline) (MCDEA). The dynamic-mechanical properties and morphological changes of sPS-(DGEBA/MCDEA) compatibilized with different amount of PS-b-PEO have been investigated in this paper. The addition of the block copolymer produced significant changes in the morphologies generated. The size of the dispersed spherical sPS spherulites does not change significantly, but less spherulites of sPS appeared upon network formation in the systems with compatibilizer, what means that addition of compatibilizer in this system delayed crystallization of sPS in sPS-(DGEBA/MCDEA) systems and change phase separation mechanism from crystallization-induced phase separation (CIPS) and reaction-induced phase separation (RIPS) almost only to RIPS. Moreover, PS-b-PEO with higher molecular weight of PS block seems to be a more effective compatibilizer than one with lower molecular weight of PS block. (c) 2006 Wiley Periodicals, Inc.