Process for cross-linking a polymer or a resin with the help of a peroxide, in the presence of a co-agent, the co-agent used being a compound according to formula 1, in which n is higher than or equal to 2, R¹ is an aliphatic or aromatic group and the R² and R³ groups, independently of each other, can be a hydrogen atom, an aliphatic or an aromatic group. The polymer used preferably is EPDM or CSM.