The interfacial tension gamma of the hexane solution of oleyl alcohol against water was measured as a function of temperature T and molality m(1) under atmospheric pressure. The entropy change associated with the adsorption triangles was dependent on both temperature and molality below about 35 mmol kg(-1) while independent of both those above about 35 mmol kg-1. The former is responsible for the contact of the double bond of oleyl alcohol with water at the hexane/water interface, but the latter is responsible for the similarity of the aggregates, which are formed by the alcohol molecules in their hexane solution, to the adsorbed films in the situation that hydrogen bonds are formed between the alcohol molecules. Considering the aggregate formation and the thermodynamic equation used, it was found that the decrease of the interfacial density Gamma(1)(H) at a high concentration region is an artifact introduced by the assumption of the ideal solution at that region. Furthermore, by drawing the interfacial pressure T versus the mean area per adsorbed molecule A curves, the onset of the phase transition comes out at high temperatures and also oleyl alcohol does not form the condensed film because of the steric hindrance of the hydrocarbon chain of the alcohol molecules. The experiments by using other oils suggested that the alkene systems obviously exhibit the phase transition. Taking notice of the affinty between water and pi-electrons and the occupied area just below and above the phase transition, it was concluded that the phase transition in this case is accompanied by the detachment of the double bond of the alcohol molecules from the interface, and therefore the driving force is the water-a-electrons interaction.