Synchrotron x-ray reflectivity is used to study the structure of a monolayer of F(CF2)(10)(CH2)(2)OH self-assembled at the liquid-liquid interface from a solution in hexane placed in contact with water. It is demonstrated that this monolayer is in a high density (solid) phase below a transition temperature. This is in contrast to the conventional expectation that soluble surfactants form disordered monolayers at the liquid-liquid interface. Above the transition temperature the monolayer desorbs into the hexane solution, leaving behind an interface with a very low density of surfactants. Hysteresis in the formation of the monolayer occurs when the temperature is scanned through the transition temperature. The success of these measurements relied upon the development of a novel technique to flatten the liquid-liquid interface to the extent required for x-ray reflectivity. The measurements of F(CF2)(10)(CH2)(2)OH at the liquid-liquid interface are compared to x-ray surface diffraction measurements of monolayers of the same material spread at the water-vapor interface. A solid to disordered-phase phase transition also occurs in the spread monolayer though at a slightly higher temperature. This indicates that the hexane acts to disorder the solid monolayer at the water-hexane interface. A measurement of the thermal expansion coefficient of the monolayer at the water-vapor interface is consistent with literature values for bulk hydrocarbon rotator phases, in contrast with previous measurements on monolayers of perfluoro-n-eicosane supported on water.