A novel method of evaluating SANS curves is presented which for the first time allows the simultaneous determination of the internal geometry and hydration of single bilayers in unilamellar vesicles at high water excess. A multistrip model unambiguously defines the water-free hydrophobic core of the membrane as a region where the neutron scattering length density equals that of a typical hydrocarbon chain. Further, the outer water-accessible parts of the membrane are defined as regions where the neutron scattering length density differs from those of pure hydrocarbons and from water. Three independent structural parameters (thickness c of the water-free core, thickness h of the water-accessible coat of the bilayer membrane, and the relationship between the scattering length densities of these membrane regions) are directly obtained by fitting the scattering curve in the q range from 0.06 nm(-1) to 3.82 nm(-1). The surface requirements of the amphiphiles and the number of water molecules located in the membrane can be calculated from these parameters. If there are multilamellar vesicles, one obtains additionally the repeat distance and the percentage of these vesicles. The potential of the method is demonstrated by elucidating structural and hydration parameters of mixed unilamellar POPC/C12E4 vesicles.