A laboratory setup has been designed to create the same excess hydraulic pressure at both membrane sides. This setup has been used for the whey ultrafiltration permeate/distilled water system to determine the critical pressure at which pores of an MFFK-3 hydrophobic membrane (Vladipor, Russia) are filled with the liquid phase. This value has been found to be 0.6 MPa. On the basis of several assumptions, it has been supposed that, the membrane pores are partially filled with the liquid phase on the both sides before attaining the critical pressure. This filling leads to a reduction in the distance between the meniscuses of the "hot" and "cold" solutions inside the pore, thereby being theoretically capable of enhancing the membrane flux. The liquid entry pressure (LEP) of this membrane has been also determined, being 130 and 50 kPa for distilled water and whey ultrafiltration permeate, respectivey.