The infrared absorption of molecules which are adsorbed or near small metal clusters, is enhanced due to an electric field around the clusters. This phenomenon is known as surface enhanced FTIR spectroscopy (SEIRA). The potential of SEIRA consists in the investigation of adsorbates and of thin layers like membranes. We used SEIRA to analyze small and specific changes in the structure of biomembranes. Two SEIRA configurations were used: underlayer and overlayer. An intensity increase in the spectra was observed for both configurations. The SEIRA spectra are discussed in terms of field interactions of clusters and adsorbed material. An effective medium model is used to explain the differences in the excitation levels of the clusters in under- and overlayer configurations.