It has been shown that the capacitance, thickness and composition of black lipid films may depend strongly on the hydrocarbon solvent used in their formation. By the use of n-hexadecane, films have been formed which contain effectively no solvent and which are comparable to the leaflets of the mesomorphic phase of the pure lipid. These films have capacitances of ca. 0.6 μF/cm(2) and hydrocarbon thicknesses of ca. 31 Å. Thinner black films of higher capacitances are also described.The capacitances of biological membranes are, in contrast, nearer to 1 μF/cm(2), and it is suggested that the hydrocarbon region in these membranes may often be thinner than in the lipid leaflets. This suggestion is consistent with some X-ray and lipid composition data. It is pointed out that if the membranes contain abnormally thin lipid leaflets, the area per polar head group of the phospholipid must be increased, and that hydrocarbon is thereby exposed to the aqueous phases. Non-polar protein residues could then interact with these hydrocarbon areas, thus tending to stabilize the expanded leaflet.