A mathematical model for the homogeneous terpolymerization of ethylene-propylene-diene (EPDM) in a semibatch reactor using Et(Ind)(2)ZrCl2/MAO as a catalyst system was developed and reported herein. In this study, we developed a kinetic model in order to explain the catalyst and EPDM properties such as catalyst activity, weight-average molecular weight, and terpolymer composition, which were experimentally and theoretically obtained. For this system, a lower E/P feed ratio leads to a lower molecular weight and a broader initial molecular weight distribution, while the increase in diene concentration leads to a decrease in the catalyst activity without broadening the MWD of the resulting polymers. The proposed model accounts for these experimental trends and for some data in the literature. (C) 2000 John Wiley & Sons, Inc.