NOVELTY - Stabilizing (S1) a semi-crystalline fluoropolymer comprising ion exchange groups involves: suspending at least one semi-crystalline fluoropolymer (I) having ion exchange groups in at least one fluorinated organic dispersing medium (M) to obtain a dispersion of polymer (I); and contacting the dispersion with fluorine under irradiation with ultraviolet (UV) radiation to obtain a stabilized polymer (II).    USE - For producing a stabilized semi-crystalline fluoropolymer having ion exchange groups useful in a membrane-electrode assembly in fuel cell devices (claimed).    ADVANTAGE - The process provides stable semi-crystalline fluoropolymers having ion exchange groups having low amount of unstable end groups such as -COF groups, thereby increasing the fuel-cell life-time; and also provides easy recovery of the stabilized material by simple separation techniques, such as decantation or filtration.    DETAILED DESCRIPTION - Stabilizing (S1) a semi-crystalline fluoropolymer comprising ion exchange groups involves: suspending at least one semi-crystalline fluoropolymer (I) having ion exchange groups in at least one fluorinated organic dispersing medium (M) to obtain a dispersion of polymer (I); and contacting the dispersion with fluorine under irradiation with ultraviolet (UV) radiation to obtain a stabilized polymer (II). The polymer (I) comprises: recurring units derived from at least one ethylenically unsaturated monomer comprising at least one fluorine atom (fluorinated monomer); and recurring units derived from at least one ethylenically unsaturated monomer comprising at least one ion exchange group (functional monomer). INDEPENDENT CLAIMS are included for the following:    (1) a stable semi-crystalline fluoropolymer (P1) having ion exchange groups comprising: recurring units derived from at least one ethylenically unsaturated monomer comprising at least one fluorine atom (fluorinated monomer); and recurring units derived from at least one ethylenically unsaturated monomer comprising at least one ion exchange group (functional monomer), where the amount of the unstable end groups of -COF type is less than 0.05 mmol/kg;    (2) a membrane comprising the stabilized semi-crystalline fluoropolymer (II) comprising ion exchange groups obtained by the process (S1), or the stable polymer (P1); and    (3) a membrane-electrode assembly (MEA) comprising the stabilized semi-crystalline fluoropolymer (II), or the stable polymer (P1).