ABSTRACT: The thermotropic phase behavior of binary mixtures of dimyristoylphosphatidylcholine with dimyristoyl glycerol ( DMPC-DMG ) has been studied in aqueous dispersion by using differential scanning calorimetry and spin label electron spin resonance spectroscopy. Phase identifications have been made by means of 31P nuclear magnetic resonance spectroscopy and x ray diffraction. The binary phase diagram of DMPC DMG mixtures displays three regions corresponding to the existence of compounds (C1 and C2, respectively) with approximately 1:1 and 1:2 mol/mol DMPC:DMG stoichiometries. The first region displays immiscibility between DMPC and C1 in the low temperature lamellar phase and miscibility of the components in the fluid phase that is lamellar. The second region displays immiscibility between Cl and C2 in the low temperature phase that is lamellar, whereas the fluid phase is of the inverted hexagonal type ( Hll) . The third region displays immiscibility between C2 and DMG in the low temperature phase that is lamellar, whereas the fluid phase is isotropic. The presence of immiscible DMG in the low temperature phase of the third region is indicated by hysteresis in the temperature scans corresponding to conversion between the stable and metastable crystalline polymorphs. Analysis of the first region of the phase diagram using regular solution theory further demonstrates the existence of a DMPC:DMG complex with approximately 1:1 stoichiometry and provides parameters for the nonideality of mixing in the fluid phase.