The ability of a MgCl2 support to activate a transition metal catalyst has been found to depend both on the crystallographic structure of the support and on the nature of the catalyst. A high degree of crystallographic disorder can be very effective for the immobilization and activation of titanium and vanadium complexes, but is not necessarily effective for zirconocene activation. A highly disordered support prepared by the reaction of MgBu2 with HCl gave high activity with TiCl4 but low activity with (n-PrCp)2ZrCl2. High polymerization activities with the zirconocene were only obtained with supports of type MgCl2/AlRn(OEt)3-n prepared from the reaction of AlR3 with MgCl2 · 1.1EtOH. These supports are characterized by additional peaks in the X-ray diffraction pattern, indicating the presence of a crystalline structure which is absent in the other supports and contains highly Lewis acidic sites able to generate the active metallocenium species.