Promoters are described for vanadium-based catalysts used in the (co)polymerization of olefins and, in particular, in the production of ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) elastomeric copolymers. The promoters of the present invention belong to the group of compounds having general formula I: ##STR00001## wherein: "X" represents a (--CO--) carbonyl or (--SO.sub.2--) sulfonyl group; "n" is 0 or 1; R' is an alkyl or alkylaryl group having from 1 to 20 carbon atoms; R''=R', H.