Fourier transform infrared transmission (FTIR/TS), external reflection (FTIR/ERS), and internal reflection (FTIR/IRS) spectroscopies are three important sampling techniques for the study of adsorbed surfactants. The optical and spectral characteristics of a three-phase system were calculated using theoretical simulation and discussed based on experimental results for oleate adsorption at the air/water interface and at the water/fluorite interface. It is shown that a thorough understanding of the optical properties and spectral characteristics from FTIR analysis helps to improve the experimental design and explanation of experimental results and is important to properly quantify surfactant interfacial adsorption phenomena.