Communications: Bimodal polyethylenes were obtained with the dual cite Cp*2ZrCl2(1)/Et(IndH4)(2)ZrCl2(2) metallocene catalyst system with a mixture of methylaluminoxane (MAO) and trimethylaluminium (TMA) as the cocatalyst. Polymer properties can be controlled by the amount of TMA added. monomer pressure, polymerization temperature and the addition of hexene or hydrogen. TMA is suggest to be partly coordinated to the active sites, thereby enhancing termination (1),increasing comonomer incorporations (2),but also blocking coordination and chain transfer to hydrogen. For the ansa catalyst, hydrogen probably relieves dormant (beta -agostic) sites.