By combining the polarization modulation (PM) technique with Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS), one can substantially improve the detection sensitivity and data collection efficiency. Using PM, the step of "background subtraction" can be eliminated for quantifying adsorbed species or covalently attached monolayers, and the IR spectra can be taken in situ in aqueous solutions. We demonstrate here that such a combination of PM and FT-IRRAS is possible, not only for external reflection measurements, but also for the attenuated total internal reflection (ATR) arrangement. The unique advantage of this second combination is that it couples the surface sensitivity of ATR with the experimental convenience and rapid data collection capability of polarization modulated FT-IRRAS. In the present work, we combine numerical calculations with experiments to study octadecylmercaptan (ODM) monolayers, self-assembled onto a gold-coated calcium fluoride prism. Results, including IR spectra (2800-3000 cm(-1)) of ODM on Au are presented for PM as well as for parallel (p) and perpendicular (s) polarization selective detection schemes and for both external reflection and ATR experimental geometries.