The deformation and fracture behavior of in reactor produced heterophasic copolymers, comprising a polypropylene (PP) matrix and an ethylene propylene copolymer (EPC) dispersed phase, have been studied as a function of the dispersed phase composition (ethylene/propylene ratio). Conventional and instrumented Charpy as well as instrumented drop weight tests were employed to quantify the response of the materials to impact loading. Scanning and high-voltage electron microscopy was used for characterization of the deformation mechanisms. Decreasing ethylene content of the EPC led to an enhancement of the matrix/dispersed phase compatibility, reduction of the dispersed phase particle size and therewith to a systematic increase of the impact strength at room temperature and a decrease of the brittle-to-tough transition temperature (T-BTT) of the materials. The low temperature impact strength was predominantly dependent upon the glass transition temperature of the EPC phase. The results are discussed from the viewpoint of interfacial interactions, size and spatial packing of the dispersed phase domains and the observed deformation mechanisms. (c) 2005 Elsevier Ltd. All rights reserved.