The phase transition in the adsorbed film of 1-icosanol (C20OH) and 1,1,2,2-tetrahydroheptadecafluorodecanol (FC10OH) mixture at the hexane/water interface was studied by measuring the pressure dependence of interfacial tension at a specific composition of FC10OH X-2 = 0.280 and 298.15 K and evaluating the volume change of adsorption. By drawing the interfacial pressure pi vs mean area per adsorbed molecule A curve, it was clarified that three types of first-order phase transitions (gaseous-expanded, expanded-FC10OH condensed, and FC10OH condensed-C20OH condensed) take place in the adsorbed film. The phase transition from the FC10OH condensed to the C20OH condensed state was accompanied by a decrease of the volume change associated with adsorption Delta nu. By evaluating the partial molar volume change of adsorption, <(nu)over bar>(H)(i)-nu(i)(O), and comparing its pressure dependence between the FC10OH condensed and C20OH condensed states, it was suggested that the pressure dependence of the partial molar volume is in the order of partial derivative nu(2)(O)/partial derivative p > partial derivative nu(1)(O)/partial derivative p > partial derivative<(nu)over bar>(H)(2)/partial derivative p approximate to partial derivative<(nu)over bar>(H)(1)/partial derivative p. Furthermore, it was found that the transition pressure pi(eq) of the FC10OH condensed-C20OH condensed transition (second transition) increases and the corresponding one of the expanded-FC10OH condensed transition (first transition) decreases with increasing pressure. This difference was explained by means of the <(nu)over bar>(H)(i)-nu(i)(O) values at the phase transition point.