Thermosetting polymer blends composed of polybenzoxazine (PBA-a) and poly(ethylene oxide) (PEO) were prepared via in situ curing reaction of benzoxazine (BA-a) in the presence of PEO, which started from the initially homogeneous mixtures of BA-a and PEO. Before curing, the BA-a/PEO blends displayed the single and composition-dependant glass transition temperatures (T-g's) in the entire blend composition, and the equilibrium melting point depression was also observed in the blends. It is judged that the BA-a/PEO blends are completely miscible. The miscibility was mainly ascribed to the contribution of entropy to mixing free energy since the molecular weight of BA-a is rather low. However, phase separation occurred after curing reaction at the elevated temperature, which was confirmed by differential scanning calorimetry (DSC) and scanning electronic microscopy (SEM). It was expected that the PBA-a/PEO blends would be miscible since PBA-a possesses a great number of phenolic hydroxyls in the molecular backbone, which are potential to form the intermolecular hydrogen bonding interactions with oxygen atoms of PEO and thus would fulfill the miscibility of the blends. To interpret the experimental results, we investigated the variable temperature Fourier transform infrared spectroscopy (FTIR) of the blends via model compound. The FTIR results indicate that the phenolic hydroxyl groups could not form the efficient intermolecular hydrogen bonding interactions at the elevated temperatures (e.g. the curing temperatures), i.e. the phenolic hydroxyl groups existed mainly in the non-associated form in the system. Therefore, the decrease of the mixing entropy still dominates the phase behavior of thermosetting blends at the elevated temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.