Dielectric properties of three curing epoxy resin systems at an industrial microwave frequency, 2.45 GHz, were measured over a temperature range lower than the cure temperature. Extent of cure, which is determined by DSC, is used to describe the progress of the polymerization. It has been found that, normally, the real and the imaginary part of the complex dielectric constant increased with temperature and decreased during microwave cure. The changes of the dielectric properties during the reaction are related to the decreasing number of the dipolar groups in the reactants and the increasing viscosity. The Davidson-Cole model can be used to describe the experimental data. The Zong model is applicable to polymeric materials at high microwave frequencies and can be used to calculate the parameters of the Davidson-Cole model. The epoxy resins exhibit one gamma relaxation, which can be described by the Arrhenius rate law. The evolutions of the parameters in the models are discussed. (c) 2005 Elsevier Ltd. All rights reserved.