We developed equilibrium phase diagrams corresponding to aqueous lipid compositions of upper small intestinal contents during lipid digestion and absorption in adult human beings. Ternary lipid systems were composed of a physiological mixture of bile salts (BS), mixed intestinal lipids (MIL), principally partially ionized fatty (oleic) acid (FA) plus racemic monooleylglycerol (MG), and cholesterol (Ch), all at fixed aqueous-electrolyte concentrations, pH, temperature, and pressure. The condensed phase diagram for typical physiological conditions (1 g/dL total lipids, FA:MG molar ratio of 5:1, pH 6.5, 0.15 M Na+ at 37 degrees C) was similar to that of a dilute model bile [BS/lecithin (PL)/Ch] system [Carey, M. C., & Small, D. M. (1978) J. Clin. Invest. 61, 998-1026]. We identified two one-phase zones composed of mixed micelles and lamellar liquid crystals, respectively, and two two-phase zones, one composed of Ch monohydrate crystals and Ch-saturated micelles and the other of physiologic relevance composed of Ch- and MIL-saturated mixed micelles and unilamellar vesicles. A single large three-phase zone in the system was composed of Ch-saturated micelles, Ch monohydrate crystals, and liquid crystals. Micellar phase boundaries for otherwise typical physiological conditions were expanded by increases in total lipid concentration (0.25-5 g/dL), pH (5.5-7.5), and FA:MG molar ratio (5-20:1), resulting in a reduction of the size of the physiological two-phase zone. Mean particle hydrodynamic radii (Rh), measured by quasielastic light scattering (QLS), demonstrated an abrupt increase from micellar (less than 40 A) to micelle plus vesicle sizes (400-700 A) as this two-phase zone was entered. With relative lipid compositions within this zone, unilamellar vesicles formed spontaneously following coprecipitation, and their sizes changed markedly as functions of time, reaching equilibrium values only after 4 days. Further, vesicle Rh values were influenced appreciably by MIL:mixed bile salt (MBS) ratio, pH, total lipid concentration, and FA:MG ratio, but not by Ch content. In comparison, micellar systems equilibrated rapidly, and their Rh values only slightly influenced by physical-chemical variables of physiological importance. In contrast to the BS-PL-Ch system [Mazer, N. A., & Carey, M. C. (1983) Biochemistry 22, 426-442], no divergence in micellar sizes occurred as the micellar phase boundary was approached. The ionization state of FA at simulated "intestinal" pH values (5.5-7.5) in the micellar and physiologic two-phase zones was principally that of 1:1 sodium hydrogen dioleate, an insoluble swelling "acid soap" compound. By phase separation and analysis, tie-lines for the constituent phase in the two-phase zone demonstrated that the mixed micelles were saturated with MIL and Ch and the coexisting vesicles were saturated with MBS, but not with Ch.(ABSTRACT TRUNCATED AT 400 WORDS)