Optical techniques play an increasingly important role in the characterization of microstructure and surface densities of thin films at various interfaces. In this study, ellipsometry and infrared reflection absorption spectroscopy (IRRAS) were used for determining the surface densities of adsorbed layers of cationic surfactants in situ at the air-water interface. The surfactants were N-alpha-lauroyl-arginine methyl ester (LAM) and N-alpha,N-omega-bis(N-alpha-lauroyl-arginine)-alpha,omega -alkylidenediamide (C-6(LA)(2)) In ellipsometry, the ellipsometric phase angle Delta was obtained at various surfactant concentrations and was referenced to that of the solvent. Three algorithms were used for analyzing the data. The surface densities are 3.3 +/- 0.3 x 10(-6) mol/m(2) at 1 mM for LAM and 1.5 +/- 0.3 x 10(-6) mol/m(2) at 0.1 mM for Cb(LA)2 by using an algorithm for which the monolayer thickness was estimated from molecular modeling. The corresponding surface densities from literature surface tension data and the Gibbs adsorption isotherm procedure are 2.2 +/- 0.4 x 10(-6) mol/m(2) and 1.2 +/- 0.2 x 10(-6) mol/m(2), respectively. In addition, IRRAS spectra were obtained from monolayers of LAM and Cr,(LA)2 at the air-water interface. The frequencies of the methylene stretching vibration bands indicate that the monolayers are liquid-like. The surface densities were determined from the reflectance-absorbance data by using the model of either an isotropic film or an anisotropic film on the aqueous subphase. The IRRAS-based surface densities from either model, by using DPPC monolayers for calibration, are 2.4 +/- 0.7 x 10(-6) mol/m2 at 1 mM for LAM and 1.5 +/- 0.6 x 10(-6) mol/m(2) at 0.1 mM for Cs(LA)2, which are in fair agreement with the ellipsometry- and the surface-tension-based surface densities.