Let us present a new automatic system which couples a high power circuit (2.45 GHz) for heating with a low power one, for broad frequency band measurements of dielectric permittivity (1 MHz-10 GHz). The sample under microwave curing consists in an epoxy resin + hardener complex (DGEBA+DDS). The broad band spectra shows the existence of a dielectric relaxation. This absorption is used as a marker to describe the evolution of the reaction in relation with time and temperature, in terms of conversion ratio, kinetic velocity and quality. The phenomenum of microwave activation and interactions between permanent molecular dipoles and electromagnetic waves form the subject of this study. We also try to establish the specificity of microwave curing from a microscopic point of view. This work emphasizes new concepts which concern microwave chemistry.