Planar bilayer membranes were formed from monolayers of a series of mono-unsaturated monoglycerides and lecithins. The hydrocarbon thickness of these membranes, as calculated from the electrical capacity, increases with the length of the fatty acid chain. The specific capacity of monoolein bilayers was found to be 0.745 muF/cm-2 which is nearly twice that of a monoolein black film made in the presence of decane, but is close to that obtained after freezing out the solvent from the black film. The hydrocarbon thickness of the bilayer, as calculated with a dielectric constant of 2.1, is considerably less than twice the length of the extended hydrocarbon chain of the monoglyceride. The specific capacity (Cm) of bilayers made from monoolein monolayers showed a negligible voltage dependence, whereas the Cm increased significantly at a voltage of 150 mV in the case of Mueller-Rudin-type monoolein films with n-decane as a solvent.