Abstract <p>The paper presents the results of interpretation and analysis of IR-ATR spectra of composite ultrafiltration (UF) membranes made of polysulfone (PS) and polyethersulfone (PES) in air-dry and water-saturated states in order to assess structural changes in the active layer caused by static and dynamic water saturation. A comparative analysis of the IR spectra of air-dry and water-saturated samples revealed that no shifts occur in the region of skeletal oscillations of the frequency of absorption bands of the functional groups of PS and PES. This allows us to affirm the stability of the chemical structure of the active layer matrix of membranes of this type. Manifestation in the IR-ATR spectra of UF membranes in the air-dry state of a wide absorption band at ~3305 cm^–1 suggests the presence of a superposition of various OH^– groups in the active layer of the PS and PES involved in the formation of <i>N</i>-dimensional hydrogen bonds. Relative changes in the shape and intensity of the band on the high-frequency side of the spectrum for water-saturated samples conceal the redistribution of hydrogen bonds of adsorbed water and OH^— groups of polyethylene glycol (PEG) and suggest that the structure of water in the pore space of the active layer of the membrane changes significantly, demonstrating an increase in weakly bound “liquid water.” The decrease in the number of PEG molecules is regular, most likely due to significant hydration of PEG molecules in adsorbed water, and indicates partial washing out (leaching), but not complete disappearance.</p>