A detailed analysis of the features Of C-18-alkylthiol/gold (silver) spectra is presented to interpret the results obtained by infrared spectroscopic ellipsometry (IRSE). The interactions between neighboring vibrational bands, the effect of different incident angles, and the Berreman effect are discussed to clarify assignments, intensities, and shifts of. absorption peaks. IRSE spectra of octadecanethiol self-assembled monolayers on gold and silver films were measured at different angles of incidence. The symmetric and asymmetric methylene and methyl stretching vibrations show no frequency variation with increasing angle of incidence of the IR beam. The relative tan psi(f)/tan psi(0) spectroscopy. The results suggest that weak bands of organic films on metals can be treated with the Lorentz-oscillator model, taking into account the coincidence of parallel and perpendicular modes and the surface selection rule. (C) 2007 American Vacuum Society.