The changes in absorbance produced when liposomes are subject to increasing osmotic pressures were correlated with the distance at which the undulation, hydration and steric repulsions dominate. It is found that at low pressures, when the bilayers are apart by more than 1 nm, the absorbance decreases with the decrease in the bilayer distance. However, at higher pressures where the bilayer are in contact within 0.7 nm the absorbance increases with the increase in pressure. This is well explained by the scattering law for particles of diameter comparable to the wavelength and fits with the empirical Bangham's law used for permeability assays. At much higher pressures, a break in the absorbance at 0.5 nm of the interbilayer distance denotes that absorbance is sensitive to the perturbation when steric forces dominate. These effects were compared to those obtained with solutes that may replace water at the membrane interface by hydrogen bonding. The results indicate that the membrane approach produces a similar effect to sucrose on both calorimetric and optical properties, suggesting that the bilayer interaction promotes a partial dehydration or reorganization of the water at the interface. The relevance of these findings on the permeation assays done with vesicles and cells by means of light scattering in which the bilayers are considered unperturbed is discussed.