The invention pertains to a process for stabilizing a semi-crystalline fluoropolymer comprising ion exchange groups, said process comprising: suspending at least one semi-crystalline fluoropolymer having ion exchange groups [polymer (I)] in at least one fluorinated organic dispersing medium [medium (M)] so as to obtain a dispersion (D) of polymer (I) in a dispersing medium (M); contacting said dispersion (D) with fluorine under irradiation with U.V. radiation so as to obtain a stabilized polymer. Still objects of the invention are a stable semi-crystalline fluoropolymer as above defined having a low amount of unstable ends groups of —COF type, the use of the stabilized fluoropolymer obtained by the process or of the stable fluoropolymer in fuel cells devices, a membrane and a membrane-electrode assembly comprising said fluoropolymers.