Separation characteristics of charged ultrafiltration membranes (UF) premodified with the anionic surfactant - sodium dodecylsulfate - were studied. We investigated the membranes characterized by the irreversible adsorption of the anionic surfactant: negatively charged membranes PA-100(-) with the pore radius of 5.7-9.0 nm and also membranes PA-100(-) with the pore radius of 2.9-5.8 nm having a small positive charge on the surface. Objects of investigation also included unmodified PA-100(-) and PA-10 ultrafiltration membranes and nanofiltration membranes OFMN-P, OPMN-K, and OPMN-KM3. We have established that due to the modification of UF membranes with the anionic surfactant their surface properties and pore structure undergo changes. The presented data indicate that the value of adsorption of the anionic surfactant affects the separation characteristics of modified membranes. Our investigations have shown that volume flow and Na+ retention coefficient of modified ultrafiltration membranes PA-10 featuring the maximum anionic surfactant adsorption are commensurable with the separation characteristics of nanofiltration membranes at much lower working pressures.