Concentration profiles of water, polymer and plasticizer in the vicinity of the membrane/water interface were studied by in situ ATR-FTIR spectroscopic imaging with Cd²⁺-selective membranes as a model system. Two membrane compositions have been studied, both contained polyvinylchloride (PVC) as matrix, 2-nitrophenyl octyl ether (oNPOE) as plasticizer, Cd²⁺-selective neutral ionophore ETH 1062, and cadmium tetrakis (p-Cl-phenyl)borate as ion-exchanger. One of the membranes contained also a lipophilic electrolyte ETH 500. Prior to measurements, the membranes with overall thickness of 700 μm were equilibrated with water for 1 week. It is shown that in the membranes the surface layers with a thickness of 150 μm are enriched with water, whereas in the bulk of the membranes practically no water was detected. Overall water uptake and also kinetics of evaporation of absorbed water have been studied gravimetrically. Much slower process of leak of the membrane components to water was studied by UV–Vis absorption spectroscopy and gravimetrically, and ion-exchanger and plasticizer are identified as leaking components.