The total lipid fraction of egg yolk (PL) and that containing p-nonylphenol (ethylene oxide) (EO-9) (molar ratio PL/EO-9 = 47 : 1) or hexane phosphonic acid diethyl ester (PAEe) (molar ratio PL/PAE 6 = 2 : 1) were studied as a function of heavy water content in a wide concentration range from 3 to 100 tool water/mol lipid by small-angle neutron scattering. The hydration capacities n h found are: 11 (PL), 12 (PL/EO-9), 21 (PL/PAE6) mol water/mol lipid and the maximum swelling n s is reached at: 23 (PL), 30 (PL/EO-9), 42 (PL/PAE6) mol water/mol lipid, respectively. The hydration and swelling properties of PL which is mainly a mixture of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were found to be the same as that of PC or PE alone. One lipid molecule in PL requires art area of 0.48 nm 2 in the 'dry' and 0.61 nm 2 in the hydrated membrane. Already a small amount of EO-9 added increases n s considerably because of th~ decrease of the chemical potential of the water by the ethylene oxide groups whereas PAE 6 induces an increase of n h and n s mainly by increasing the accessibility of the lipids to water. Additionally, the PAE 6 induces a considerable decrease of the thickness of the lipid bilayer, which can be understood by an interdigitation of the lipids. Finally, indications are found that the forces between the membranes are dependent on the curvature of the membranes.