The formation of finely monodispersed oil-in-water (O/W) emulsions under high temperature and pressure conditions (240 degrees C and 10 MPa) depended on the solubility of the oil in water. Fatty acids with large solubilities in water favored the formation of oil droplets in emulsions of less than 100 nm in diameter. The water dissolved in the oil droplets participated throughout the cooling process, followed by the break-down of the oil droplets. In contrast, organic solvents with lower solubilities in water formed oil droplets in emulsions that had at most a diameter of 300 nm.