The ~ potential of the membrane surface of lipid emulsion {LE) has been measured. In order to analyze the experimental results, a theory is proposed for the relationship between the surface potential and the surface charge density of a lipid membrane of LEs in mixed solutions of 1-1 and 2-1 electrolytes. We have introduced the concept of "'an effective charge" of the lipid membrane surface with no net charge. LEs composed of egg yolk lecithin with/ without oleic acid (PC99LE: (99% phosphatidylcholine (PC)); PC99OLE (99% PC and oleic acid): PC70LE (70% PC)) have negative charges in aqueous solution at pH > 4. Results of theoretical analysis showed that the areas per binding site S' for Ca 2+ or Mg 2+ are equal to the areas per effective charge on all LEs, and the values of S' for Ca 2+ are the same as those for Mg 2+. The binding constants of LEs for Ca 2+ and Mg 2+ were 646 and 528 M 1 119 and 72 M 1 and 143 and 64 M 1 on the lipid surface of PC99LE, PC99OLE and PC70LE, respectively. These findings suggest that the purity of PC and addition of oleic acid influence the adsorption of divalent cations.