The kinetics of ethylene-norbomene copolymerization to cyclic olefin copolymer has been investigated using homogeneous rac-Et(1-indenyl)(2)ZrCl2/methylaluminoxane catalyst in toluene at 70 degreesC for norbornene/ethylene mole ratios in the range of 3-55. It is shown that the catalyst is ineffective for the homopolymerization of norbornene, but the catalyst activity for the insertion of norbornene into a growing polymer chain increases dramatically in the presence of ethylene. A terminal model has been developed to quantify the kinetic behavior of the copolymerization process, but it was found inadequate to represent the copolymerization kinetics for a broad range of polymerization conditions. To overcome the limitations of the terminal model we developed a penultimate model and excellent agreement between the experimental data and model calculations has been obtained. In particular, it is shown that the penultimate model accurately predicts the effect of bulk phase norbornene concentrations on the polymerization rate.