Infrared and 31P-NMR spectra of aqueous dispersions of 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine (DMPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS), 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS) and ox brain phosphatidylserine in the presence of excess Mg2+ have been recorded. A consistent picture emerges from the application of infrared and 31P-NMR spectroscopy to Mg2+-PS interactions. Mg2+ forms crystalline complexes with saturated phosphatidylserines, such as DMPS, and probably with POPS. In these crystalline PS-Mg2+ complexes the phosphate group loses its water of hydration but the serine carboxylate remains hydrated. Furthermore, there is formation of an additional hydrogen bond to one of the ester carbonyl group of DMPS, and interchain interactions appear to be enhanced as reflected by a tighter packing of the fatty acyl chains. One main conclusion of this work is that Mg2+ binding to PS bilayers shows a gradation, the binding is in the order DMPS > POPS > ox brain PS > DOPS. The molecular area increases in the order DMPS < ox brain PS < POPS < DOPS and is apparently an important parameter determining the affinity of PS for Mg2+. The general trend is that with increasing molecular area, and hence spacing of the ligands, the binding of Mg2+ decreases. While PS with two saturated fatty acyl chains forms tightly packed, crystalline Mg2+ complexes with an immobilized headgroup, the unsaturated PS molecules such as ox brain PS and DOPS interact only weakly with Mg2+. Their interaction seems to be restricted to electrostatic shielding, since no major changes in molecular conformation, chain packing and headgroup hydration are found. The interaction of POPS with Mg2+ is intermediate between that of saturated PS and that of DOPS. POPS exhibits a higher affinity for Mg2+ than ox brain PS, although their molecular areas (and the surface charge density) are approximately the same. This apparent anomaly is proposed to be due to a discreteness of charge effect. It is proposed that a lipid surface with regularly spaced polar groups has a higher affinity for binding Mg2+.