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 to be unsuitable for explaining the polymer blend morphology. At a PET concentration of 20%, the extrudate consists of three regions. The skin layer, which is about 10 mum thick, has a lower concentration of the dispersed PET phase than the overall concentration. The intermediate region, which is about 400 mum thick, has profuse PET fibers and some small PET particles. The central region, which is approximately 800 tLm in diameter, mainly contains 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, the variation of the barrel temperature, die temperature, and cooling media did not produce a significant affect on the PET particle size distribution 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 extrudates through elongation of 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. J Appl Polym Sci 89: 1743-1752, 2003.