An internal reflection spectroscopy (IRS) element may be modified by application of thin layers of metal oxides, aramids or carbonised polymers, so that it resembles chemically the surface of reinforcements (eg. glass, carbon, Kevlar) used in the production of reinforced plastics. The first 0.2-1 µm (depending on wavelength) of polymer matrix adjacent to the modified surface may then be examined by FT-IR, using "windows" in the spectra of the coatings. In this way the simulated reinforcement surfaces are shown to modify the chemistry of the thin layer of polymer matrix adjacent to the surface (ie. the interphase). These chemical differences are shown to correlate with property differences in reinforced plastics.