The influence on the thermal diffusivity of the changes induced on the macromolecular network of an epoxy-amine system by different hardener/resin ratios was experimentally investigated. For each hardener/resin ratio, the open photoacoustic cell technique was used to measure the thermal diffusivity. The thermal diffusivity data was correlated with the evolution of the characteristic infrared absorption bands of the epoxy and amine components using a fast Fourier transform spectrometer. The results show a sharp transition in the thermal diffusivity values at the beginning of the amine-rich domain, corresponding to a transformation from a rigid network to an amine saturated deformable structure. It is concluded that the open photoacoustic cell technique can be a useful monitoring tool for structural characterization and molecular evolution during the curing of this kind of system.