A crosslinking system of perfuoroelastomers curable by peroxidic way comprising: as crosslinking agent, a bis-olefin having general formula: wherein: R1, R2, R3, R4, R5, R6, equal to or different fromeach other, are H or C1 -C5 alkyls; Z is a C1 -C18 linear or branched alkylene or cycloalkylene radical, optionally containing oxygen atoms, or a (per)fluoropolyoxyalkylene radical; the bis-olefin amount, expressed in per cent by weight on the total of polymer, being from 0.6% to 1.8%; peroxides for the peroxidic curing, capable to generate radicals by heating, in an amount from 0.2% to 1.3% by weight on the polymer.