The kinetics of dissolution of dispersions of egg phosphatidylcholine (lecithin) by bile salts WBS studied by observing the decrease in turbidity as mixed micelles of lecithin and bile salts were formed. The rate of dissolution of lecithin corresponding to formation of mixed micelles was studied in the presence of dihydroxy bile salts, sodium deoxycholate, sodium chenodeoxycholate, sodium ursodeoxycholate and one trihydroxy bile salt, sodium cholate. The rate of dissolution of lecithin and mixed micelle formation decreased in the order: chenodeoxycholate > deoxycholate cholate > ursodeoxycholate. Kinetic solvent isotope studies in DzO, along with measurement of enthalpies of mixed micelle formation both in HzO and DzO, suggest that formation and stabilization of mixed micelles are related to "hydrophobicity" as estimated by high performance liquid chromatography retention factors.On the basis of phase equilibrium and X-ray diffraction studies, a generally accepted model for the bile saltlecithin mixed micelle was proposed by Small (1, 2) and Dervichian (3). According to this model, a disc-shaped micelle is formed on the association of lecithin with bile salt. The disc core consists of hydrocarbon alkyl chains of the lecithin molecules surrounded by a ring of bile salt molecules. Thus, the disc-shaped micelle exterior presents to the aqueous solvent only the hydrophilic end groups of the lecithin and the hydrophilic sides of the bile salt.Whereas it is generally agreed that mixed micelles occur in these solutions, there remains controversy concerning their detailed structure. Mazer and coworkers (4) have suggested that their quasielastic light scattering data (QLS) can best be interpreted in terms of a mixed disc model. This model preserves the disc-shaped mixed micelle of Small and Dervichian but also requires the presence of some bile salt molecules within the interior of the micelle. These bile salt molecules within the micelle interior are presumed to exist as hydrogen-bonded dimers. Calorimetrically derived exothermic heats of mixed micelle formation have also been interpreted to support this model (5). Recently, Muller (6) has proposed another modification to the mixed micelle model. On the basis of X-ray small angle scattering data, Muller suggests that structural dimorphism exists in bile salt leci- The purpose of this study was to attempt to shed further light on the process of mixed micelle formation by measuring the rate of dissolution of lecithin in bile salt solutions.The kinetics of mixed micelle formation were followed by studying the time-dependent decrease in turbidities of a dispersion of lecithin (L) after addition of a given bile salt (BS) solution (9). In the course of the reaction multilamellar L interacts with BS to form mixed micelles M L + B S e M