The crystallisation behaviour and morphology of binary phase separated crystalline/amorphous blends of poly(ethylene oxide) (PEO) and poly(ether sulphone) (PES) are investigated. The crystallisation behaviour of the PEO/PES blends changes strongly with demixing time and temperature. A method is proposed to determine the composition and amount of PEO-rich phase in the partially demixed blend systems from the dynamic crystallisation behaviour, The results are in agreement with the structure development as predicted for spinodal decomposition that proves the validity of this method. The phase and semi-crystalline morphologies are visualised by scanning electron microscopy and atomic force microscopy. The semi-crystalline morphology is also studied by real-time small angle and wide angle X-ray scattering. It is shown that, depending on the blend composition, a spherulitic or isolated lamellar crystalline morphology is formed in the demixed PEO/PES blends. (C) 1999 Elsevier Science Ltd. All rights reserved.