Infrared ellipsometry has been applied to determine the refractive index n and the absorption index k as well as the thickness of metal island films causing surface enhanced infrared absorption (SEIRA). The results from numerous films prepared in several campaigns are presented. For films of a nominal thickness of 6 nm, k is found to range from 0 to 3, while n ranges from 5 to 8. Among these two optical constants a close correlation is observed. Layers suitable for SEIRA exhibit an unusual spectral feature whose origin is explained.