X‐ray reflectivity studies of surfactants at the water‐oil interface yield structural information with sub‐nanometer resolution. In this presentation, we reviewed recent X‐ray reflectivity measurements of the interface between water and a hexane solution of the hydrocarbon alkanol CH3(CH2)19OH and fluorocarbon alkanol CF3(CF2)7(CH2)2OH. The mixed system exhibits three monolayer phases, two of which are similar to single surfactant phases. A transition from a liquid monolayer to a solid monolayer occurs with increasing temperature. This unusual phase transition and the qualitative features of the phase diagram are predicted by an appropriate superposition of the behavior of the two single surfactant systems.