We investigated the relationship between morphology and mechanical properties in binary blends of isotactic polypropylene (PP) and atactic polystyrene (PS) showing completely phase-separated morphology. The domain size and crystallinity in PP phase were controlled by the comp-molding conditions such as holding time in the hot-press and quenching temperature to prepare the sheets. The phase-inversion took place at 62.5 wt % of PS content and this leads to ductile-brittle transition at the blend composition. It was found that the mechanical properties of PP-rich blends are dominantly affected by the nature of the PP matrix and independent of the domain size of PS phase. This is associated with the interfacial separation between rigid PS droplets and PP matrix. (C) 2008 Wiley Periodicals, Inc.