Ultrafine polyamide-6 (UPA6) with a size of 4-8 mu m was prepared via jet-milling. Blends of poly (vinyl chloride) (PVC) and UPA6 using a reactive copolymer styrene-maleic anhydride (SMA-18 %) were prepared. The change in morphology and structure of the blends were studied using differential scanning calorimetry, scanning electron microscopy, and X-ray diffraction. The blend behavior was also determined experimentally using dynamic mechanical analysis. Contrasted to the original PA6, the crystallinity of the UPA6 decreased, the size of its crystallites were reduced, and its melting point decreased to 175 degrees C. In all blends, PVC formed the continuous matrix phase. SMA is miscible with PVC and tends to be dissolved in the PVC phase during the earlier stages of blending. The dissolved SMA has the opportunity to react with PA6 at the interface to form the desirable SMA-g-PA6 copolymer. This in situ formed SMA-g-PA6 graft copolymer tends to anchor along the interface to reduce the interfacial tension and results in finer phase domains. Cocrystallity existed in PVC/(UPA6/ SMA) at a ratio of 82/(18/5). (c) 2005 Wiley Periodicals, Inc.