The structure-property relationships of isotactic polypropylene (iPP)/styrenic block copolymer blends filled with talc were examined by optical and scanning electron microscopy, wide-angle X-ray diffraction, and tensile- and impact strength measurements. The composites were analyzed as a function of the poly(styrene-b-ethylene-co-propylene) diblock copolymer (SEP) and the poly(styrene-b-butadiene-b-styrene) triblock copolymer (SBS) content in the range from 0 to 20 vol % as elastomeric components and with 12 vol % of aminosilane surface-treated talc as a filler. Talc crystals incorporated in the iPP matrix accommodated mostly plane-parallel to the. surface of the samples and strongly affected the crystallization process of the iPP matrix. The SBS block copolymer disoriented plane-parallel talc crystals more significantly than the SEP block copolymer. The mechanical properties depended on the final phase morphology of the investigated iPP blends and composites and supermolecular structure of the iPP matrix because of the interactivity between their components. (C) 2004 Wiley Periodicals, Inc.