Bile salt-lecithin mixed micellar solutions are the components of mammalian bile chiefly responsible for the lipid solubilizing property of this important biological solution. Differing theories for the structure of bile salt-lecithin micelles have been proposed. It will be the objective of this research to obtain thermodynamic data for these mixed micellar solutions against which the proposed theories may be tested. Recent calorimetric measurements of the heat accompanying micelle formation have shown that in all cases, heat is evolved when complex bile salt-lecithin micelles are formed. It has been suggested that this heat is due to the pairwise association by hydrogen bonding of bile salt molecules to each other within the micelle. The theory that bile salt molecules associate on partitioning into the lipid interior of the micelle will be tested in several ways. Thermodynamic properties for the partitioning of bile salt molecules into organic solvents resembling the micelle interior will be determined and compared with the thermodynamic parameters obtained for micellar association. Properties to be determined include equilibrium partition coefficients and enthalpy of partition. Further studies on the mixed micellar solutions will include the measurement of heat capacities and densities. It is anticipated that these studies will elucidate the nature of these important bile salt-lecithin mixed micellar solutions.