Poly(ethylene terephthalate) /polypropylene (PET/PP) blends of different compositions were extruded through a 2-mm capillary die using a corotating twin-screw extruder. The extrudates were cryogenically fractured and examined using scanning electron microscopy. The viscosity ratio of the constituent polymers alone was found not suitable for explaining the polymer blend morphology. At a PET concentration of 20%, the extrudate consists of three regions: The skin layer, about 10 mum thick, has a lower concentration of the dispersed PET phase than that of the overall concentration. The intermediate region, about 400 fun thick, has profuse PET fibers and some small PET particles. The central region, approximately 800 mum in diameter, contains mainly PET particles that are generally bigger. A low barrel temperature, low die temperature, and fast cooling rate helped to retain the fibers near the extrudate skin. Meanwhile, variation of the barrel temperature, die temperature, and cooling media did not affect the PET particle-size distribution significantly in the central region of the extrudate. A high screw speed and a high postextrusion drawing speed were very effective in producing fibers in the extruclates through elongation of the particles. At a PET concentration of 30%, coalescence of the PET phase was prevalent, leading to the formation of PET platelets near the extrudate skin and irregular PET networks in the central region of the extrudate. (C) 2003 Wiley Periodicals, Inc.