The solubilization of dimyristoylphosphatidylcholine (DMPC) vesicles by the surfactant sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB), respectively, was studied by isothermal titration calorimetry (ITC) at 30 and 60 degreesC and at different lipid concentrations. The partitioning of surfactant into lipid bilayers and the following solubilization of vesicles show a temperature dependence due to hydrophobic interactions, which is similar to temperature dependence of the cmc of surfactants. From the ITC data, we constructed phase diagrams in the total surfactant vs lipid concentration and surfactant concentration vs temperature plane, respectively. The micellar, the coexistence, and the vesicular region can not only be reached by concentration changes but also by simply raising the temperature of a lipid-surfactant mixture with a fixed concentration. From these phase diagrams, the transition temperature for a particular lipid-surfactant mixture can be predicted. The predictions were verified by following the temperature-induced formation of fluid vesicles from mixed micelles upon heating a lipid-surfactant mixture. The micelle-vesicle transitions of mixtures of DMPC with SDS or DTAB in water and the reverse transition to mixed micelles observed upon cooling was studied by differential scanning calorimetry (DSC), turbidity measurements using UV-vis spectrophotometry, and dynamic light scattering methods. The endothermic effects observed in the DSC curves for the micelle-vesicle transition are a consequence of the endothermic heats of transfer of surfactant as well as lipid from a mixed micelle to a mixed vesicle as determined by ITC.