Abstract The interaction of lanthanides and other cations with phosphatidylcholine bilayers present as single bilayer vesicles in 2H2O has been investigated in terms of stoichiometry, apparent binding constants and environmental conditions. Lanthanides are shown to form 2 : 1 (molar ratio) phosphatidylcholine to metal ion complexes. The apparent binding constant K b varies as a function of the quantity of metal ion bound and as a function of the Cl− concentration. The apparent binding constant at “zero loading” is K 0 = 1.25 · 10 4 L 2 · M − at 0.15 M KCl . It decreases exponentially with increased “loading” expressed as the molar ratio of metal ion bound to effective phosphatidylcholine concentration and increases exponential with Cl− concentration. The interaction of lanthanides and divalent cations such as Ca2+ and Mg2+ is independent of pH in the pH range 3–7+ and 3–10 respectively, but is sensitive to the nature of the anion. The presence of anions enhances the interaction with polyvalent cations, the chaotropic anions showing the largest effect. The order of enhancement is Cl − − 3 − − − 4 − . The nature of the monovalent counterion (cation) has little effect on the enhanced binding of lanthanides in the presence of the above anions. The affinity of other polyvalent cations for phosphatidylcholine bilayers has been determined by competition with lanthanides. The physiologically important divalent cations Ca2+ and Mg2+ both bind less strongly (by about an order of magnitude) to the lipid surface. The order of binding of cations reflects direct binding to the phosphodiester group, with UO22+ showing the highest affinity.