Sulphonic fluorinated ionomers crosslinked by radical crosslinking of: A) crosslinkable sulphonic fluorinated ionomers, having equivalent weight 380 1300 g/eq and comprising: from 48% to 85% by moles of monomeric units deriving from tetrafluoroethylene (TFE); from 15% to 47% by moles of fluorinated monomeric units containing sulphonyl groups --SO.sub.2F; from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula: ##STR00001## wherein: m=2 10, preferably 4 8; R.sub.1, R.sub.2, R.sub.5, R.sub.6, equal to or different from each other, are H or C.sub.1 C.sub.5 alkyl groups; B) a fluorinated compound as crosslinking radical initiator; C) a fluorinated bis-olefin of structure (I) as crosslinking agent; the radical crosslinking being carried out at a temperature in the range 250.degree. C. 310.degree. C., preferably 260.degree. C. 300.degree. C.