Abstract: A process for the preparation of heterophasic polyolefin compositions by (i) polymerizing to form (a) a crystalline propylene or ethylene polymer, and (b) an elastomeric copolymer of ethylene and a C3-C10 &agr;-olefin, wherein the polymerizations are carried out in the presence of a Ziegler-Natta catalyst, (ii) contacting the polymer obtained in (i) with: (a) a compound capable of deactivating the catalyst present in (i), and (b) a catalyst (2) obtained by contacting (I) a compound containing a transition metal M, and at least one ligand coordinated with the metal M by a &pgr; bond, and (II) a cocatalyst, and (iii) copolymerizing ethylene and a C3-C10 &agr;-olefin in the presence of the polymer treated as in (ii), and the catalyst (2).