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.sbd.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.degree. C) than the Na+ or NH4+ salts of DMPS (39.degree. C). 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.sbd.O ester bonds of the phosphate group which remain in the gauche.sbd.gauche conformation; and (3) the packing of the fatty acyl chains becomes more ordered. In addition, the properties of the DMPS.sbd.Ca2+ complex were studied by infrared spectroscopy. While the DMPS.sbd.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.sbd.Ca2+ [Casal, H. L., Mantsch, H. H., Paltauf, F., and Hauser, H. (1987) Biochim. Biophys. Acta (in press)], the phosphate group undergoes a conformational change, probably to the antiplanar.sbd.antiplanar conformation, and loses its water of hydration. In contrast to the DMPS.sbd.Li+ complex, the carboxylate group remains hydrated in the DMPS.sbd.Ca2+ complex, indicating that Ca2+ is chelated by phosphate groups only. Furthermore in the DMPS.sbd.Ca2+ complex, one of the ester carbonyl groups is engaged in hydrogen bonding; such a hydrogen bond is not found in DMPS.sbd.Na+, DMPS.sbd.NH4+, and DMPS.sbd.Li+.