The interfacial tension gamma of the hexane solution of 1H, 1H-perfluorononanol (FDFC9OH) and its omega-hydrogenated analogue, 1H,1H,9H-perfluorononanol (HDFC9OH), against water was measured as a function of pressure and concentration at 298.15 K in order to clarify the effect of omega-dipole on the orientation of fluorononanol molecules from the viewpoint of volume. The adsorbed films of both alcohols exhibit two kinds of phase transitions among three different states: the gaseous, expanded, and condensed states. The partial molar volume changes of adsorption nu(1)(-H) - nu(1)(O) in the expanded and condensed states were evaluated and compared between the two systems. The nu(1)(-H) - nu(1)(O) values of both alcohols are negative, and thus the alcohol molecules have smaller volume in the adsorbed film than in the bulk solution. Furthermore, the nu(1)(-H) value was obtained through the evaluation of nu(1)(O) by the density measurement of the bulk hexane solution. It was found that the nu(1)(-H) value of HDFC9OH is smaller than that of FDFC9OH in the condensed state. On the basis of three matters concerning the molecular structure of alcohols, the occupied area at the interface, and the orientation of FDFC9OH in the adsorbed film deduced from the earlier results of X-ray reflectivity measurement, the mean tilt angle of HDFC9OH from the interface normal in the condensed film was estimated to be 15 degrees. The thermodynamic estimation demonstrated here is highly valuable one to provide structure information on an adsorbed film.