Abstract: A grafting reaction system comprises an activated polymer and a grafting medium containing a monomer, with the system containing free radicals. The system further comprises 0.1 to 2% by weight of the grafting medium and monomer of a homopolymerization inhibitor compound, the inhibitor compound comprising a compatabilizing function toward the grafting medium which is selected from the group consisting of sulfonic, carboxylic and phenolic groups, and an inhibiting function which acts as a transfer agent for the free radicals, the inhibiting function being an allyl-type unsaturated group. The inhibitor compound has a diffusing capacity through the grafting medium which is greater than its diffusing capacity through the polymer.