The phase transition of an 8CB monolayer on a water surface was investigated experimentally and theoretically. A weak first-order phase transition of the nematic molecules from isotropic to polar orientation was clearly detected by compressing the film in the range of immeasurably low surface pressure by means of Maxwell-displacement-current-measuring techniques. On the basis of the interaction of dipolar molecules with a liquid surface, the thermodynamics of the molecular orientation and related mechanical and electric effects for Langmuir monolayers of molecules were also studied theoretically to elucidate the experimental results. It was shown that the compression of the molecular area should lead to a weak first-order phase transition of the orientation of molecules from the isotropic phase to the polar one. It was also predicted theoretically that a Maxwell displacement current with a sharp pulse shape is generated at, the onset of the transition.