The presence of an organic solvent often modifies the transport properties of an ion exchange membrane. Compared to ethylene glycol (EG) and N-methylformamide (NMF), tetrahydrofuran (THF) has a particular behavior in a Nafion perfluorosulfonic membrane. This solvent induces a higher dimensional expansion but without significant modification of the transport properties in a 20% v/v THF-water mixture. Raman spectroscopy enables the location of the THF molecules in the membrane material. The hydrophobic part of the solvent molecules is in the perfluorinated chains (inducing a high dimensional expansion) while the hydrophilic part interacts with the aqueous clusters. Three water molecules in the range of 20-80% v/v THF-water surround each THF molecule.