Partially crystalline thermosetting epoxy polymers of defined heterogeneous morphology are herein presented. Crystalline poly(e-caprolactone) (PCL) domains are covalently integrated into the cationically polymerized epoxy network. Although crystallinity is common in thermoplastic polymers, these new materials are among the first examples of partially crystalline thermosetting polymers. Nucleation is crucial and leads to a defined heterogeneous morphology. Homogeneous and heterogeneous polymers with identical composition can be prepared by initiation below or above the melting point of the PCL, respectively. While the homogeneous polymer showed the expected decrease in crosslinking density of the epoxy network and low glass transition temperature (T-g), the epoxy phase is not substantially influenced in the case of the heterogeneous morphology, showing two separated modulus changes at the melting point of PCL and the T-g of the epoxy matrix polymer. (c) 2013 Elsevier Ltd. All rights reserved.