Several types of commercial and developmental aluminoxanes were studied in the semi-batch polymerization of ethylene using bis(cyclopentadienyl) zirconium dichloride (Cp2ZrCl2). High temperature gel permeation chromatography (GPC) measurements of polyethylene (PE) formed using various types of aluminoxanes approximated Flory＇s most probable distribution from a single site-type with PE M-w/M(n)s approaching 2.0 for polymerization temperatures ranging from 50 to 90 degrees C. The structure and type of aluminoxane co-catalyst were found not to influence the molecular weight distribution (MWD) of PE although to influence the activity of the Cp2ZrCl2 catalyst. In general, modified methylaluminoxanes (MMAOs) and methylaluminoxanes (MAOs) were found to have similar activities, whereas isobutylaluminoxane (IBAO) gave a very low activity in the polymerization of ethylene. Increasing isobutyl content in the aluminoxane co-catalyst led to a continual decrease in catalyst activity. Increasing aluminium/zirconium (Al/Zr) molar ratio, by increasing the MAO concentration, led to increasing catalyst activity up to a maximum activity at an Al/Zr molar ratio of 2400. Like the structure and type of the aluminoxane co-catalyst, the Al/Zr molar ratio was found not to influence the PE MWD. Mixing in additional trimethylaluminium (TMA) with MAO was found not to alter the MWD but gave lower activity at similar Al/Zr molar ratios than that from MAO itself.