External reflection infrared spectroscopy has been applied to the in situ measurement of the infrared spectra of insoluble monomolecular films that have been spread at the air-water interface. Theoretical calculations were made using classical electromagnetic theory for an N-phase system of parallel, optically isotropic layers. The macroscopic electromagnetic properties of a hypothetical monolayer organic film at the air-water interface were compared with those at the air-metal interface. It is shown that, by utilizing the reflectance properties of water, finite values of the mean square electric fields for both parallel and perpendicular polarizations are present at the air-water interface, in constrast with the air-metal phase boundary, where only parallel polarized vibrations may be observed. Theoretical calculations of the reflection-absorbance for a monolayer on water suggest that the optimal angle for experimental determination of the reflection spectra of thin films on water is in the range 0-40°, unlike that on metal, where the optimal angle is near grazing incidence. Further calculations show that the theoretical absorbances expected for a monolayer film on water are within 60-100% of the maximum obtainable absorbance of a monolayer film on metal, although the absorbance bands will appear negative due to the properties of the complex refractive index of water. Experimental spectra of 1,2-distearoyl-sn-glycero-3-phosphocholine as a monolayer on water are presented for both parallel and perpendicular polarizations in the C-H stretching region. © 1986 American Chemical Society.