The results of the study of a completely miscible thermosetting polymer blend containing a crystallizable component are reported. Blends of poly(epsilon -caprolactone) (PCL) and bisphenol-a-type epoxy resin (ER) cured with 4,4'-methylenebis(3-chloro-2,6-diethylan (MCDEA) were prepared and compared with blends of PCL with uncured bisphenol A-type epoxy resin, i.e. diglycidylether of bisphenol A (DGEBA). The miscibility, crystallization behavior and spherulitic morphology of both uncured DGEBA/PCL blends and MCDEA-cured ER/PCL blends were investigated by differential scanning calorimetry (DSC), optical microscopy, and Fourier-transform infrared (FTIR) spectroscopy. It was found that PCL is completely miscible with both DGEBA and MCDEA-cured ER in the melt and in the amorphous state over the entire composition range, as shown by the existence of a single composition-dependent glass transition temperature (T-g). The overall crystallization rate and crystallinity of PCL in the MCDEA-cured ER/PCL blends decrease much more rapidly with increasing amorphous content than those of the DGEBA/PCL blends. The spherulitic morphology of PCL in both the uncured and the cured blends is characteristic of miscible crystalline/amorphous blends. and the PCL spherulites in these blends are always completely volume-filling. The miscibility of the uncured DGEBA/PCL blends is considered to be due predominately to the entropic contribution, whereas that of the cured ER/PCL blends is due to the enthalpic contribution. FTIR investigations indicated hydrogen bonding interaction between the hydroxyl groups of MCDEA-cured ER and the carbonyl groups of PCL in the cured system, which is an important driving force for the miscibility of the cured ER/PCL blends. Real-time small-angle X-ray scattering (SAXS) experiments revealed that the amorphous cured ER segregated interlamellarly during the crystallization process of PCL, which is considered to result from the low chain mobility of the cured ER in the ER/PCL blends. On the basis of the SAXS results, a model describing the semicrystalline morphology of MCDEA-cured ER/PCL. blends is proposed. The amorphous fraction of PCL, the branched ER chains and imperfect ER network are located between PCL lamellae in the crystallized blend. (C) 2001 Elsevier Science Ltd. All rights reserved.