The infrared optical properties of diamond films on silicon substrates, grown by means of microwave plasma CVD method, are studied by infrared spectroscopic ellipsometry in the spectral range of 2.5-12.5 mum. It has been found that the establishment of appropriate models has the strongest influence on the fit of ellipsometric spectra. The best fit is achieved for a diamond film with a 77.5 nm middle layer of SiO2 included by Bruggeman EMA. Finally, the refractive index (n) and the extinctive coefficient (k) are calculated for the diamond films. (C) 2004 Elsevier B.V. All rights reserved.