A number of studies on thermosetting polymeric composite materials have indicated the existence of a three dimensional region of the matrix in the vicinity of the fiber, which exhibits distinct properties. A key factor in the interphase formation of epoxy-diamine mixtures is the diffusivity of the diamine relative to the epoxy prepolymer. A technique for measuring diffusion coefficients of a liquid through solid polymeric thin layers is described. Infrared attenuated total reflectance (ATR) spectroscopy is used to analyze diffusion couples with infrared distinguishable bands. A cylindrical germanium crystal, coated with the polymeric layer of interest, is immersed in a boat cell filled with the diffusant, and infrared spectra of the system are taken in real time. By those means, only the region close to the solid surface is scanned without interferences from the bulk. The temperature dependence of diffusivity can also be evaluated by the use of a heated cell. Diffusivities that are evaluated in this way can be used to predict stoichiometric gradients during the curing reaction of the system and, therefore, the structure and property gradients of the material around the fiber.