An investigation was conducted of the effects of the advancement of chemical reactions and the prepolymer molecular weight on the reorientational dynamics and intermolecular cooperativity in reactive epoxy-amine systems. Experimental results were obtained by dielectric spectroscopy over a wide range of frequency (1 mHz to 1 GHz) and temperature (−170 to +200 °C). A strong effect of the progress of reaction on reorientational dynamics was noted and, for the first time, an explanation was put forward within the framework of the coupling theory. It was proposed that the molecular-level characteristics of a reactive system that affect its intermolecular cooperativity can be classified into two categories: 1) molecular architecture - determined by molecular symmetry, rigidity and steric hindrance; and 2) dielectric architecture - determined by the type and concentration of all dielectrically active species.