Cryo-transmission electron microscopy was used to investigate the aggregate structure in dilute aqueous samples of oleic acid, as a function of pH. At pH 10.7, where the fatty acid is almost completely deprotonated, the micrographs show spherical or cylindrical micelles depending on the concentration. Upon a decrease of the pH to values just above 9, formation of unilamellar vesicles is induced. With decreasing pH the vesicles show an increasing tendency to aggregate. At pH between 8 and 7, large clusters of aggregated vesicles coexist with dispersed nonlamellar, presumably inverted hexagonal structures. Further decrease in pH results in a complete transition into nonlamellar liquid-crystalline structures and finally to the formation of oil droplets. Addition of high concentrations of oleic acid to small unilamellar lecithin vesicles induces, at pH 7.4 and lower, clustering and formation of particles with nonlamellar structure. At high pH, on the other hand, oleic acid acts like a conventional cationic surfactant. With increasing fatty acid : lipid molar ratio both significant vesicle growth and finally lipid solubilization into mixed micelles are observed.