Infrared spectroscopic ellipsometry was applied to determine the optical constants of metallic island films which are used to generate surface enhanced infrared absorption (SEIRA) of adsorbates. It was found that their optical constants differ considerably from those of the bulk metal. The optical properties of the island film as well as the enhancement of the spectral response of an adsorbate are reproduced with an effective medium approach. The simulations reveal two contributions to the enhancement.