A nonlinear constitutive equation has been proposed to describe the dependence of the cross-link density vs. peroxide and coagent concentrations, following the dynamic vulcanization of ethene-propene copolymers (EPM) rubbers. This was obtained by integrating the rate equations of the fundamental reactions of the cross-linking processes (beta-scission, chemical coupling, coagent grafting). The compression set test ASTM D 395 was then modeled and put in correlation with the cross-linking density. The model was then validated with compression set experimental data, obtained from a series of EPM rubbers dynamically cross-linked under various conditions of peroxide/coagent content. The two most meaningful model parameters (N-1 and N-2, which are linked to the order of the rate equations) have been correlated with the elastomer microstructure, particularly with the ethene intramolecular chain distribution and content. (c) 2005 Society of Plastics Engineers.