The dihedral angle of an alcohol lens floating on the air/water interface and the three kinds of interfacial tensions of air/1-undecanol/water and air/1-dodecanol/water systems were measured as a function of temperature under atmospheric pressure. By applying the thermodynamics of interfaces to the experimental results of the interfacial tension measurement, it was found that the phase transitions between the expanded and the condensed states take place in the interfaces. It was found that there are break points on the dihedral angle versus temperature curves corresponding to the phase transitions of the interfacial films. The mutual relation among the states and the phase transition of the interfacial film, the wetting behavior, and the intruding phenomenon of the water phase on the air/alcohol interface was discussed. It was shown that the dihedral angles measured coincide with those calculated by Neumann＇s equation with a satisfactory accuracy except at very low temperatures. The discrepancy at the low temperatures wets proved to be attributable to the meniscus of the air/water interface.