Monolayer films of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air-water (A/W) interface have been investigated in situ using external reflection-absorption IR spectroscopy. The IR spectra of the monolayer films were monitored as a function of the two-dimensional molecular area of the phospholipd molecules on the surface of a Langmuir trough. Examination of the conformation-sensitive CH stretching bands of the molecule's hydrocarbon chains reveals the existence of two phase transitions during film compression. The first transition corresponds to the main liquid-expanded-to-liquid-condensed monolayer phase transition (at ∼58-78 Å2 molecule-1), while the second transition occurs at ∼40-57 Å2 molecule-1 and is indicative of a transition to a solid-condensed phase under increasing pressure. The headgroup bands from the phosphate ester group were also monitored during film compression. The asymmetric phosphate stretching band of the DPPC monolayer film is composed of two bands, one at ∼1220 cm-1 and the second at ∼1257 cm-1. These bands are also seen, and frequency shifts noted, for the interaction of the soluble cations Ca2+ and Pr3+ with the phospholipid phosphate group. The band splitting and frequency shifts suggest a mixture of hydration states for the lipid's phosphate group. The equilibrium between these hydration states can be changed by film compression and/or cation interaction with the lipid headgroup. © 1989.