The hydration repulsive force between lipid bilayers and the deformability of both gel and liquid-crystalline bilayers have been quantitated by an X-ray diffraction analysis of osmotically stressed liposomes. Both sampling theorem reconstructions and electron density distributions were calculated from diffraction data obtained from multilayers with applied osmotic pressures of 0-50 atm. The bilayer thickness and area per lipid molecule remain nearly constant (to within about 4%) in this pressure range, as adjacent bilayers move from their equilibrium separation in excess water to within 2-4 A of each other. This analysis indicates that the bilayers are relatively incompressible. This results differs from previously published X-ray diffraction studies of bilayer compressibility but agrees with direct mechanical measurements of the bilayer compressibility modulus. It is also found that the hydration repulsive force decays exponentially with separation between bilayers with a decay constant of 1.4 A for gel-state dipalmitoylphosphatidylcholine and 1.7 A for liquid-crystalline egg phosphatidylcholine bilayers. This implies that the exponential decay constant is not necessarily equal to the diameter of a water molecule, as has been previously suggested on experimental and theoretical grounds.