The interaction of submicellar concentrations of various physiologically important unconjugated [sodium deoxycholate (NaDC), sodium cholate (NaC)] and conjugated [sodium glycodeoxycholate (NaGDC), sodium glycocholate (NaGC), sodium taurodeoxycholate (NaTDC), sodium taurocholate (NaTC)] bile salts with dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) small unilamellar vesicles in solid gel (SG) and liquid crystalline (LC) phases was investigated using the excited-state prototropism of 1-naphthol. Steady-state and time-resolved fluorescence of the two excited-state prototropic forms of 1-naphthol indicate that submicellar bile salt concentration induces hydration of the lipid bilayer membrane into the core region. This hydration effect is a general phenomenon of the bile salts studied. The bilayer hydration efficiency of the bile salt follows the order NaDC > NaC > NaGDC > NaTDC > NaGC > NaTC for both DPPC and DMPC vesicles in their SG and LC phases.