The foaming of films of a fluorinated ethylene propylene copolymer (FEP) using supercritical carbon dioxide (scCO(2)) was investigated. For this purpose, a one-step foaming process was applied to the films inside a pressure vessel at temperatures in excess of 200 degrees C and pressures up to 30 MPa. The films prepared show a homogeneous cellular structure with thin compact polymer layers on both sides. The foaming behaviour of the FEP films is determined in dependence on the exposure time of the polymer to the scCO(2), the processing temperature and pressure as well as on the pressure drop rate during the foaming process. The resulting density and the foam morphology can be related to the pressure level during the saturation process. The size of the cells generated mainly depends on the foaming temperature, whereas their number per volume is determined by the magnitude of the applied pressure. Furthermore, the pressure drop rate influences the size of the cells in a way that significantly fewer but larger bubbles are found for longer times of depressurizing. This finding can be explained by nucleation rates becoming the smaller the slower the rate of pressure decrease. (C) 2008 Elsevier B.V. All rights reserved.