Abstract: Catalysts for the preparation of copolymers of ethylene and/or alpha-olefins and optionally dienes, are obtained by reacting an Al-alkyl with a solid catalytic component comprising:(A) a magnesium compound containing at least an Mg-halogen bond;(B) a halogenated compound of at least partly trivalent Ti or V, containing at least a Ti-OR or V-OR bond, wherein the Mg/Ti(V), halogen/Ti(V), halogen/Mg and OR/Ti(V) atomic ratios are within determined ranges; and(C) an electron-donor compound.Elastomeric copolymers endowed with improved tension set values are obtained with said catalysts.