The structure of expanded poly(tetrafluoroethylene) (ePTFE) films that were produced by uniaxial or biaxial stretching of a calendared sheet were studied by wide angle X-ray diffraction (WAXD), small angle X-ray scattering, differential scanning calorimetry (DSC), and scanning electron microscopy. The molecular orientation of the stretched films was analyzed by WAXD flat films and pole figures. Biaxial orientation factors were computed to interpret the level of orientation quantitatively. DSC scans showed that oriented samples exhibited two melting peaks, one at the commonly observed temperature in the range 340-345 degrees C and one around 380 degrees C. The possible causes of this high-temperature melting peak and its relation to previously described processes is discussed. The microporous nature of the ePTFE films is also briefly discussed. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2248-2256, 2010