The interfacial tension, gamma, of the hexane solution of the 1-icosanol (C20OH) and 1,1,2,2-tetrahydroheptadecafluorodecanol (FC10OH) mixture against water was measured as a function of total molality in and composition of FC10OH, X-2, at 298.15 K under atmospheric pressure. All the gamma vs m curves have a break point (first break) that corresponds to the phase transition from the expanded to the condensed state. Furthermore, it was found that the curves at X-2 = 0.275 and 0.280 show another break (second break) at a high concentration. By calculating the interfacial density TH and then drawing the interfacial pressure, pi, vs area per adsorbed molecule, A, curves, it was suggested that the first-order phase transition takes place from the expanded to the condensed film of C20OH at X-2 ( 0.250 and to that of FC10OH at X-2 > 0.300 at the first break point. Furthermore the second break point was suggested to correspond to the first-order phase transition from the FC10OH condensed film to the C20OH condensed film. These findings were confirmed by the interfacial densities of individual components and support that C20OH and FC10OH molecules are completely immiscible in the condensed film, although they are miscible with each other at all proportions in the expanded film. Furthermore, it is shown that the phase transition from the C20OH to the FC10OH condensed state does not take place at the second break point because the cross-sectional area of the fluorocarbon chain is larger than that of the hydrocarbon chain and the condensed film is constructed of individual alcohol molecules.