Large multilamellar vesicles have been prepared from dipalmitoylphosphatidylcholine (DPPC)—phosphatidylinositol (PI) mixtures over the complete range of composition. The electrophoretic mobilities of the vesicles have been measured in a buffer at pH 6.6 and as a function of lanthanum ion concentration over a range covering the zero point of charge. The zeta potentials of the vesicles increase with PI content and the degree of ionisation of the PI molecules lies in the range 0.16 to 0.23. The zeta potentials have been interpreted in terms of La 3+ ion binding using a Gouy—Chapman—Stern model as proposed by Ottewill and co-workers. Apparent and intrinsic binding constants and the number of binding sites have been determined. The Gibbs energy of ion adsorption (Δ G 0 ads) has been found to obey the linear relationship: Δ G 0 ads (kJ mol −1) = −(66.9 (± 1.7) x PI + 41.2 (± 0.8) x DPPC) which demonstrates that each phospholipid molecule binds La 3+ ions independently.