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 —SO₂F; from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula: ##STR1##

wherein: m=2-10, preferably 4-8; R₁, R₂, R₅, R₆, equal to or different from each other, are H or C₁-C₅ 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 C.-310 C., preferably 260 C.-300 C.