Poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) (Teflon(R) PFA) membranes of a variety of structures have been produced through thermally induced phase separation of Teflon(R) PFA-chlorotrifluoroethylene melt-blends of different compositions. A phase diagram of the two component system was constructed, and electron microscopy was used to characterize the structures of membranes produced. The morphological characteristics of the Teflon(R) PFA membranes have been explained on the basis of equilibrium driving forces for liquid-liquid and solid-liquid phase separations.