Stable aqueous dispersions of sesame oil (SO) were obtained by co-sonication with soybean phosphatidylcholine (PC) in the SO mole fraction range of 0.1-0.8. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction of SO with PC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 40-60 nm. The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker, calcein. The trapped volume in the SO/PC particles decreased remarkably with the addtion of SO into small unilamellar vesicles of PC. The decline in fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the PC membrane by the addition of the quencher CuSO4. These results indicate that the excess SO separated from the PC bilayers is stabilized as emulsion particles by the PC surface monolayer. Monolayer-bilayer equilibrium of SO/PC mixtures was estimated by measurements of spreading and collapse pressures. The results showed that the coexistence of emulsion particles (surface monolayer of PC + core of SO) with vesicular particles (bilayer) was critically important for the formation of stably dispersed particles of the lipid mixture.