An equation to calculate surfactant adsorption density from Fourier transform infrared external reflection spectra was established. The derivation and limitation of this equation are discussed in detail. The validation of the FTIR/ERS adsorption density equation was experimentally verified from the analysis of Langmuir-Blodgett films of stearic acid with and without calcium ions in the subphase at the air-water interface and at fluorite surface. In this way the properties of Langmuir-Blodgett films at the air-water interface are further characterized using FTIR/ERS. (C) 2003 American Institute of Physics.