The ionized phospholipids NaDLPG and NaDMPG each undergo a transformation from a sponge phase to unilamellar vesicles at a critical temperature T* (Koshinuma; et al. Langmuir 1999, 15, 3430). The present study examines the dependence of TY On composition in aqueous bilayer dispersions of binary mixtures of these Lipids. The surface pressure of the saturated solutions show a maximum at T* that is a monotonic function of the lipid composition in the bilayer. The lipid composition in the equilibrium aqueous solution was also measured. At T*, and only at this temperature, the lipid composition of the bilayer and equilibrium solution become identical. These properties are shown to be a natural consequence of the surface pressure maximum at T*, and are indicative of the physical uniqueness of the critical bilayer state. In general, when aqueous dispersions of bilayer-forming multicomponent mixtures of phospholipids exhibit a surface pressure maximum at a critical temperature T*, the lipid composition of the solution and bilayer phases become identical. This property is a characteristic of the general class of indifferent states.