Some rheological properties of a perfluorosulphonated PTFE copolymer have been measured in order to characterise the melt-state viscous and elastic behaviour of a thermoplastic precursor of Nafion(R), an ion-selective membrane used in the electrochemical industry. Steady-state shear viscosity measurements show conventional pseudoplastic flow behaviour over a wide temperature range, under high shear conditions. These data have been modelled to a high level of accuracy using polynomial simulations to obtain Carreau model coefficients and flow activation energies. Using an orifice die in a capillary rheometer, calculated extensional viscosity data are shown to decrease with stress and are more temperature-sensitive than shear viscosity. Although die swell increases with shear rate in a conventional manner, unusual and complex die swell data (at a fixed shear stress) have been obtained in response to an increase in process temperature. This behaviour is attributed to the breakdown of a small-scale network of domains and ordered crystalline material in the amorphous matrix of the precursor, as exemplified by a very broad melting endotherm. Different modes of deformation have been proposed to explain the rheological data observed across the process temperature range. The observed changes to the flow mechanism and elastic character of the melt carry practical implications for the extrusion processes and developed microstructure of film products manufactured from this precursor copolymer. (C) 2005 Springer Science + Business Media, Inc.