The viscoelastic behavior of high impact polypropylene (PP) melts, a multiphase system with an isotactic PP matrix and inclusions consisting of mainly amorphous ethylene-propylene copolymer (EP), is strongly influenced by the size of small inclusions. The dynamic moduli of such two-phase systems (reactor products) are well described by Palierne's emulsion model. For this analysis the reactor product is separated into matrix and dispersed phase via the different solubility in xylene. This analysis also provides information on the ratio interfacial tension to particle size of the dispersed phase. With morphology data (particle size) of the solidified heterophasic samples (Transmission Electron Microscopy), we estimate the interfacial tension between PP and EP copolymer via the emulsion model of Palierne and from the relaxation time spectra according to Gramespacher and Meissner. (c) 2007 Wiley Periodicals, Inc.