In ethylene/propylene copolymerizations with cyclopentadienylzirconium dichloride (Cp2-ZrCl2)/methylaluminoxane (MAO) catalyst, correlations were found between polymerization temperature (-15-degrees-C to +55-degrees-C), catalyst activity, and viscosity average molecular weight. As the polymerization temperature increased, the activity of the catalyst increased and the viscosity average molecular weight of the copolymer product decreased. The activity of the catalyst was higher than that reported earlier, even with the differences in polymerization conditions taken into account. Changing the solvent from heptane to toluene had no effect on the viscosity average molecular weight of the product. In toluene the catalyst activity was higher. The higher the amount of propylene in the feed, the lower the viscosity average molecular weight. Increase in the catalyst mole ratio from 2300 to 6900 Al/Zr did not affect either the catalyst activity or the viscosity average molecular weight of the copolymer. As expected for metallocene catalyst, GPC analysis showed the molecular weight distribution of the products to be narrow.