Abstract: It is known to heat treat melt-processible blends of copolymers of tetrafluoroethylene and hexafluoropropylene in the molten state at elevated temperatures for several hours in the presence of an atmosphere containing water vapor in order to remove volatile material from the copolymer and to improve the thermal stability of the copolymer. Such heating results in foaming of the molten copolymer which is undesirable. According to the process of this invention, the foaming can be reduced by employing a blend of two or more of the copolymers, at least one of which has a melt viscosity higher, and at least one of which has a melt viscosity lower, than the final desired viscosity of the treated blend.