This work aims at studying the origin of spontaneous emulsification occurring at the oil/water interface. This phenomenon was observed for the five crude oils tested as well as at the interface of an asphaltene-toluene mixture and water. The kinetics of appearance of water microdroplets was slowed down for increasing salt concentrations, and the microdroplet formation ceases when the chemical potential of water they contain is equal to that of the water in the bulk solution. Nucleation events occur at the oil-water interface and at the solid surface/liquid interface: some water microdroplets are stuck together close to the oil/water interface, whereas others grow in oil and sediment or nucleate at the oil/solid surface. This suggests the following mechanism: water molecules diffuse from the water reservoir into the oil phase and then create droplets. These droplets are simultaneously fed by hydrosoluble "osmogeneous" species increasing the osmotic pressure, inducing an osmotic pumping of water molecules into microdroplets.