Small-angle neutron scattering was performed on a series of expanded polytetrafluoroethylene membranes that were subjected to thermal treatment at a maximum of 320degreesC at pressures up to 517 kPa. The scattering data indicate the existence of monodisperse pores of 2.7-Angstrom radius, and a polydisperse set of scatterers with surface fractal properties. The thermal treatment had no significant effect on the structure, given that the maximum temperature was below the softening point of the polymer. The effect of increasing pressure was to increase the fractal dimension and to decrease the porosity void fractions. A linear relationship was noted between the Porod invariant values and the applied pressure. The pressure treatment had no effect on the size of the 2.7-Angstrom pores. (C) 2003 Wiley Periodicals, Inc.