The fluorescence spectrum of 1-(4-dimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (DMA-DPH) dissolved in a stoichiometric mixture of diglycidyl ether of bisphenol A and diethylene triamine was measured as a function of cure time at various cure temperatures. The frequency of the fluorescence maximum for DMA-DPH increased during the curing reactions because of the change in the polarity of the epoxy resin. In an isothermal cure, the fluorescence frequency increased linearly with the cure time until the gelation occurred. The total change in fluorescence frequency that occurred from the beginning of the isothermal cure to the gelation time was 1000 cm-1 and was independent of the cure temperature, implying that the chemical structure of the infinite network at the gelation time was independent of the cure temperature. The rate constant, K(T), for the polarity change during an isothermal cure of the epoxy resin, defined as the rate constant for the linear increase in fluorescence frequency, was determined. The activation energy of K(T), was estimated to be 60 kJ mol-1.