Bound water is a major component of biological membranes and is required for the structural stability of the lipid bilayer. It has also been postulated that it is involved in water transport, membrane fusion, and mobility of membrane proteins and lipids. We have measured the fluorescence emission of membrane-bound 1-anilino-8-naphthalenesulfonate (ANS) and the infrared spectra of membranes, both as a function of hydration. ANS fluorescence is sensitive to polarity and fluidity of the membrane-aqueous interface, while infrared absorption is sensitive to the hydrogen bonding and vibrational motion of water and membrane proteins and lipids. The fluorescence results provide evidence of increasing rigidity and/or decreasing polarity of the membrane-aqueous interface with removal of water. The membrane infrared spectra show prominent hydration-dependent changes in a number of bands with possible assignments to cholesterol (vinyl CH bend, OH stretch), protein (amide A, II, V), and bound water (OH stretch). Further characterization of the bound water should allow its incorporation into current models of membrane structure and give insight into the role of membrane hydration in cell surface function.