Divalent cation (Ca 2÷ and Mg 2÷) and hydrogen ion-induced fusions of small unilamellar phosphatidic acid vesicles were studied by the use of fluorescence fusion assay. These fusogenic ions also increased interracial tension and reduced the surface dielectric constant of phosphatidic acid membranes as the ion concentration increased. A good correlation was found between the threshold of vesicle fusion and the degree of changes in such membrane properties induced by these fusogenic ions. At the fusion threshold point, the increase in interracial tension and the decreased surface dielectric constant of the membrane were approximately 6 and 14 dyn/cm, respectively, regardless of Ca 2+, Mg 2÷ or H ÷. Such changes correspond to the increases in surface hydrophobicity of the membrane. As these fusogenic ionic concentrations increased, the electrophoretic mobility of the phosphatidic acid vesicle decreased, because of the binding of ions to negatively charged sites of the membranes. However, the quantities of the reduced negative surface charges were not necessarily in good correlation with the threshold of vesicle fusion. It is suggested that the complexes of the phosphate group with these fusogenic ions are responsible for increases in the surface hydrophobicity of the membrane, which is an important factor for membrane fusion.