The free-volume properties of high-impact polystyrene (HIPS)/polypropylene (PP) and HIPS/high-density polyethylene (HDPE) blends were investigated by means of positron annihilation lifetime spectroscopy (PALS). The measured results show that the free-volume holes in the semicrystalline polymers, such as PP and HDPE, were not large enough to accommodate the branched chains and the end groups of the macromolecular chains in HIPS to produce favorable interactions between the semicrystalline polymers and the HIPS polymer in these blends; thus immiscible blends were formed. The weak interaction between two dissimilar polymer molecules only took place in the regions between two amorphous phases. In addition, the observed negative deviations of the longest lifetime intensity and the free-volume fraction were attributed to the influence of the interfacial polarization during PALS measurement. (C) 2003 Wiley Periodicals, Inc.