Small-angle neutron scattering data for lipid bilayers in excess water are presented. The method of solvent contrast variation was applied. The variation of scattering intensity with the scattering angle could be analyzed in terms of the model of scattering by quasi-2-dimensional systems of O. Kratky and G. Porod (1948). From this, the bilayer thickness of unilamellar vesicles of dimyristoyllecithin was detd. to be 41 .ANG. at 310 K. Several typical mixts. of different lecithins with varying chain lengths and of lecithin with phosphatidic acid were studied. By deuteration of 1 lipid component, a very large contrast between segregated phases could be achieved. It was thus possible to distinguish clearly between homogeneous mixts. and mixts. which exhibit a heterogeneous lipid organization. In the latter case, no intensity matching on solvent contrast variation was possible. Phase boundaries were detd. very accurately by performing contrast variation expts. with at most 2 mixts. of different initial compn. The main advantage of the present method was that lateral phase sepn. could also be clearly studied in fluid states of the bilayer. From the av. scattering-length ds., the d. of the lipid layers, and thus the excess vols. of the mixts. could be detd. For lecithin mixts., pos. excess vols. were obsd., whereas for the lecithin-phosphatidic acid mixt., the excess vol. was neg. [on SciFinder (R)]