The thermotropic phase behavior of fully hydrated Na⁺ and/or NH₄⁺ salts of 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) was determined by temperature-dependent infrared spectra. The molecular level properties and thermal phase behavior of DMPS-Li⁺ complexes were also characterized by infrared spectroscopy. With increasing concentrations of Li⁺, the infrared spectra reveal the appearance of a second, more ordered, lipid phase which shows a gel to liquid-crystal transition at significantly higher temperatures (75-95 ºC) than the Na⁺ or NH4⁺ salts of DMPS (39 ºC). Li⁺ binds to the phosphate and carboxylate groups of DMPS, resulting in the following changes: (1) water of hydration is lost from both the carboxylate and phosphate groups; (2) there are changes in the conformation of the glycerol backbone but not in the P-0 ester bonds of the phosphate group which remain in the gauche-gauche conformation; and (3) the packing of the fatty acyl chains becomes more ordered. In addition, the properties of the DMPS-Ca²⁺ complex were studied by infrared spectroscopy. While the DMPS-Ca²⁺ complex is also characterized by rigidly packed, well-ordered fatty acyl chains, the mode of Ca²⁺ binding to the DMPS head groups differs significantly from that of Li⁺ binding. By comparison, with dry DMPS-Ca²⁺ [Casal, H. L., Mantsch, H. H., Paltauf, F., & Hauser, H. (1987) Biochim. Biophys. Acta (in press)], the phosphate group undergoes a conformational change, probably to the antiplanar-antiplanar conformation, and loses its water of hydration. In contrast to the DMPS-Li⁺ complex, the carboxylate group remains hydrated in the DMPS-Ca²⁺ complex, indicating that Ca²⁺ is chelated by phosphate groups only. Furthermore, in the DMPS-Ca²⁺ complex, one of the ester carbonyl groups is engaged in hydrogen bonding; such a hydrogen bond is not found in DMPS-Na⁺, DMPS-NH₄⁺, and DMPS-Li⁺.