It is shown, on the basis of the analysis of selective behavior of ultrafiltration (UF) membranes, that the change of selective properties in the course of concentration polarization (CP) (the shift of membrane retention curve towards low molecular weights) is caused by formation on the membrane surface of a gel layer. This layer which decreases the effective pore size is a superposition of adsorption and gel-polarization layers. The change in selective properties of UF membranes does not depend on solute and membrane hydrophilicities and is determined only by the CP intensity. On the contrary, the recovery of membrane selectivity characteristics, as well as the recovery of membrane permeability, is determined by the relative thickness of adsorbed polymer layer, i.e. by the hydrophilicity of solute-membrane system.