Over the past several years, this laboratory has adapted external reflection-absorption infrared spectroscopy to the study of conformation-sensitive vibrational modes in amphiphilic monolayers at the air-water (NW) interface. It has proven possible to investigate the surface pressure-induced first order thermodynamic phase transition of phospholipid monolayer films at the A/W interface using this technique. The IR results to date indicate that this phase transition is accompanied by a change in conformational order in the hydrocarbon portion of the molecule and suggests that the monolayer is heterogeneous and biphasic in character, with coexistence of molecular domains throughout the phase transition. More recently, we have explored the charged, hydrophilic head group region of the amphiphilic monolayer and the interaction of sub-phase ions and pH with the molecule's interfacial ionic groups. The spectra obtained of the head group region in the monolayer films clearly indicate that the relevant vibrational modes are sensitive to hydration and protonation of the ionic groups in the monolayer. These results strongly suggest that the external reflection technique described here has the requisite sensitivity to probe questions involving complex physical chemistry and interfacial structure in amphiphilic monolayer films at the A/W interface and may be useful in describing the detailed molecular structure of such films. © 1989 SPIE.