The current study investigates the diffusion of a bisphenol A type epoxy resin and a diamine curing agent into polysulfone (PSU) using a Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR) technique. The diffusion of epoxy or amine through the film was monitored in real time by measuring the changing intensity of selected characteristic absorbance bands in each of the polymers. The diffusion coefficients for the epoxy and amine were determined independently using the appropriate solution to the diffusion equation and the experimental data. Diffusivities were evaluated at several temperatures, and an Arrhenius relationship was observed. Results show that the diffusivity of amine is an order of magnitude greater than the diffusivity of epoxy at each temperature tested. Furthermore, the epoxy diffusion appears to be Fickian, while the diffusion of amine appears to be non-Fickian.