Fourier transform infrared (FT-IR) spectroscopy is used to investigate the complex conformational changes that occur as phosphatidylserine (PS) binds to calcium. The spectra confirm the isothermal crystallization of the hydrocarbon chains in the PS-Ca2+ complex. However, in contrast to differential scanning calorimetry, which detects no phase transitions under 100°C in PS-Ca2+ complexes, several FT-IR parameters detect structural changes at 30–40°C in these complexes analogous to those observed in solid-solid phase transitions of alkanes. Site symmetry splitting observed in the PO2− bands suggests that Ca2+ binds to the PS phosphate as a bidentate ligand; in addition, Ca2+ causes a dehydration of the phosphate ester. No evidence is found for the specific chelation of Ca2+ by the ionized carboxylate group or the dehydration of this group; instead, the carboxylate exists in an immobilized conformation in the presence of Ca2+. Splitting of the degenerate vibrations of the carbonyl group at the interfacial region suggests different rotational chain isomers in the Ca2+ complex and the possibility of hydrogen bonding with trapped interstitial water. © 1983, American Chemical Society. All rights reserved.