The invention mainly pertains to a method for heat treating a composition [composition (C)] which contains at least one melt-processible perfluoropolymer [polymer (F)] formed of tetrafluoroethylene (TFE) copolymer with one or more perfluorinated comonomers [comonomer (F)] containing at least one unsaturation of ethylene type in amounts from 0.5% to 13% by weight, preferably from 0.6% to 11% by weight, and more preferably from 0.8% to 9% by weight; said polymer (F) possessing reactive end groups comprising at least one of the group consisting of hydrogen atoms, oxygen atoms and ethylenically unsaturated double bonds in an amount of at least 4.5 mmol/kg, the process comprising at least the step of heat-treating the composition (C) at a temperature of at least 260° C.