A catalyst system comprising the product obtained by contacting (a) a solid catalyst component containing Mg, Ti, halogen and at least an electron donor compound selected from phenylene aromatic diesters of a specific formula (b) an alkylaluminum cocatalyst and (c) an ester of formula R′OOC—(CR″2)m—COOR′ in which m is an integer from 2 to 7, the R′ groups, equal to or different from each other, are C1-C10 alkyl groups and the R″ groups, independently, are hydrogen or C1-C15 hydrocarbon groups.