Abstract <p>A study was conducted of ultrafiltration separation of a water-in-oil emulsion based on I-20A oil using polyethersulfone membranes with a cut-off particle mass of 10–300 kDa. The water-in-oil emulsion composition was as follows, % by volume: oil 20 surfactant of the Neonol AF 9-10 brand 2, distilled water 78. Separation was carried out on a laboratory ultrafiltration unit in the cross-flow mode, the volume of the separated mixture was 100 cm³, the process pressure was 2 atm. The efficiency of emulsion separation and membrane productivity were determined as operational characteristics depending on the sizes of particles cut off by the membranes. The effect of applied pressure and pore size on the productivity of polyethersulfone membranes for distilled water was determined. The obtained graphical dependencies were interpolated using Lagrange polynomials and expressions describing the change in productivity values ​​from the pore size at different values ​​of the applied pressure were determined. Similar graphical dependencies of the change in membrane productivity during separation of water-in-oil emulsion at different values ​​of the applied pressure in the system were obtained and interpolated using Lagrange polynomials. The effect of the membrane pore size on the COD values ​​of the filtrates was also estimated depending on the pressure applied to the system. It was revealed that the efficiency of the ultrafiltration process decreases with increasing pressure and filter pore size, since the amount of organic components in the resulting filtrate increases according to the obtained COD values. It was concluded that the obtained analytical expressions will allow, without conducting many similar experiments, to obtain the process parameters depending on the initial data.</p>