Novel catalytic systems, prepared in situ by the oxidative addition of 8-hydroxyquinoline ligands to bis(1,5-cyclooctadiene)nickel(0) and activated by methylaluminoxane, were studied in ethylene polymerization. When 8-hydroxyquinoline was employed, only oligomeric products were obtained. On the contrary, 5,7-dinitro-8-hydroxyquinoline gave linear polyethylene (PE), but with a modest activity. For the catalyst based on 5-nitro-8-hydroxyquinoline, the productivity was largely dependent on the content of free trimethylaluminum. (TMA) present in the commercial aluminoxane. The progressive optimization of the TMA/oligomeric methylaluminoxane ratio increased the productivity, which reached 700 kg of PE/(mol of Ni x h), by an order of magnitude. (c) 2005 Wiley Periodicals, Inc.