This research work demonstrates how an optical technique, developed for studying the process of polymer swelling, is able to produce highly reproducible data of swelling of dense polymeric membranes exposed to liquid mixtures. This technique was applied for studying the swelling process of different polymeric membranes, from proton conducting polymers (Nafion) to non-charged hydrophobic polymers, such as a polyurethane-polybutadiene elastomer (PU/PBDO), polydimethylsiloxane (PDMS) and polyvinylidene fluoride membrane (PVDF). Room temperature ionic liquids (RTlLs) were chosen as solvents to be studied, due to the growing interest of using their unique physico-chemical properties in hybrid membrane processes. The results obtained show that the technique developed allows for studying the swelling and "deswelling" of polymeric membranes even when anisotropic phenomena take place. These results are discussed and interpreted from the perspective of the nanoscale properties of the polymers. (C) 2007 Elsevier B.V. All rights reserved.