Oleic acid (OA) is a monounsaturated omega-9 fatty acid which undergoes a first-order phase transition when it is influenced by high pressures. The transition results in a change of molecular structure, which was investigated by means of X-ray techniques [J. Przedmojski and R.M. Siegoczyński, X-ray diffraction investigation of oleic acid under high pressure, Phase Transit. 75 (2002), pp. 373-377]. Despite a significant change in the structure, the permittivity of the acid remains barely influenced during and after the phase transition. A rise in relative permittivity has been observed due to the increasing number of molecules per volume unit within a compression cycle. However, no significant fall in the permittivity has been observed as a result of the transition which occurs in the case of edible oils. This may be caused by a strongly dimerised structure and dipole-dipole interaction in the liquid at ambient pressure.