Revised Manuscript Received January 24, 1986 abstract: The physical properties in water of a series of 1: 1 acid-soap compounds formed from fatty acids and potassium soaps with saturated (10-18 carbons) and o>-9 monounsaturated (18 carbons) hydrocarbon chains have been studied by using differential scanning calorimetry (DSC), X-ray diffraction, and direct and polarized light microscopy. DSC showed three phase transitions corresponding to the melting of crystalline water, the melting of crystalline lipid hydrocarbon chains, and the decomposition of the 1: 1 acid-soap compound into its parent fatty acid and soap. Low-and wide-angle X-ray diffraction patterns revealed spacings that corresponded (with increasing hydration) to acid-soap crystals, hexagonal type II liquid crystals, and lamellar liquid crystals. The lamellar phase swelled from bilayer repeat distances of 68 (at 45% H20) to 303 A (at 90% H20). Direct and polarized light micrographs demonstrated the formation of myelin figures as well as birefringent optical textures corresponding to hexagonal and lamellar mesophases. Assuming that 1: 1 potassium hydrogen dioleate and water were two components, we constructed a temperaturecomposition phase diagram. Interpretation of the data using the Gibbs phase rule showed that, at> 30% water, hydrocarbon chain melting was accompanied by decomposition of the 1: 1 acid-soap compound and the system changed from a two-component to a three-component system. Comparison of hydrated 1: 1 fatty acid/soap systems with hydrated soap systems suggests that the reduced degree of charge repulsion between polar groups causes half-ionized fatty acids in excess water to form bilayers rather than micelles. The 1: 1 fatty acid/soap system provides insights into the physical states formed by free fatty acids during transport and metabolism in vivo since, at pH 7.4, fatty acids in water and in phospholipid bilayers are near halfionization. e physical properties of “fatty acids” 1 in water are largely influenced by the ionization state of the carboxyl group (Small, 1968).“Fatty acid”/water systems have been studied exten-sively at the extremes of ionization, and their physical prop-erties are well-known [for recent reviews, see Taylor and Princen (1979) and Small (1986)]. For example, fully ionized fatty acids (soaps) form micelles, liquid crystals, or crystals in water whereas fully un-ionized fatty acids form oil droplets or crystals. However, the physical states formed at intermediate degrees of ionization are not well-known, andmost of the work on these systems was done many years ago, often on impure systems.