Hydration properties of mixtures of a zwitterionic lipid, palmitoyloleoylphosphatidylcholine (POPC), and nonionic surfactants (oligo(oxyethylene) dodecyl ether, C(12)E(n) with n = 1-8) were studied over a wide range of surfactant/lipid molar ratios R(A/L) from 0.1 to 2 at T = 25 degrees C. The adsorption of water by the POPC/C(12)E(n) mixtures was measured by the isopiestic method at two different relative humidities (RH = 86.5 and 97%). Deuterium NMR on (H2O)-H-2 and P-31 NMR on the phospholipid as well as X-ray diffraction were employed to characterize the phase state of the mixtures. For samples in the L(alpha) phase the area requirement of POPC and surfactant molecules and the thickness of the hydrophobic core of the bilayer were estimated fi om the repeat spacing and the known composition of the sample. The experimental results are compared to data reported previously for pure POPC and C(12)E(n) systems underidentical conditions. Small C(12)E(n) concentrations (R(A/L) = 0.1 and 0.2) in the membrane tighten the membrane packing. The area per molecule in the bilayer/water interface occupied by the lipid is reduced and that of the surfactant enlarged in the mixture compared to bilayers of the pure components under equal conditions. Further increase of the surfactant concentration causes a significant thinning of the hydrophobic core and a progressive increase of the area requirement of the amphiphilic molecules in the membrane/water interface. Finally, at high surfactant concentrations (R(A/L) = 1 and 2) the area requirement of the amphiphilic constituents and the vertical extension of the polar interface region increase with growing ethylene oxide chain length n. The hydration of the Lipid is reduced by the presence of C(12)E(n) to a level comparable to the primary hydration shell. The first two or three oxyethylene groups next to the alkyl chain of the surfactant also show reduced hydration in the mixtures. The remaining EO groups have hydration characteristics very similar to the pure surfactant, with the exception of bilayers with R(A/L) = 2 at RH = 97%.