Phase separation in blends of crystalline poly(ethylene oxide) (PEO) and amorphous poly(methyl methacrylate) (PMMA) or poly(methyl acrylate) (PIMA) has been examined using DTA, small-angle light scattering, and polarized microscopy. The structure of the polymer blends was analysed by the wide-angle X-ray diffraction technique. It is found that phase separation takes place in both systems above the melting point (T-m) of the crystalline component. PEO/PMMA blends have an upper critical solution temperature above the glass transition temperature and T-m of the system. Components of PEO/PMMA blends are compatible in two regions: when the PMA concentration is 0.0-0.5 weight fraction or exceeds 0.8 weight fraction. PEO and Ph LA are immiscible in the melt at intermediate concentrations. It is shown that compatibility of components in the melt of crystalline and amorphous polymers is not a universal property of polymer blends. (C) 2000 Society of Chemical Industry.