Dipalmitoyl phosphatidylcholine (DPPC) and egg phosphatidylcholine (egg PC) are not completely miscible at all temperatures. Their phase diagram was determined by differential scanning calorimetry (DSC) of aqueous mixtures of the two. From the integrated DSC curves we obtained the enthalpy of solution of DPPC in egg PC, Δhs, as a function of the mole fraction of DPPC, X, and using the empirical relationship between Δhs and X, the solubility Xsat as a function of temperature, T. The latter could be described by the semiempirical relationship:RlnXsat = a + blnT – c/T, where a = 6.57 × 10−2 kcal∙mol−1∙degree−1 and c = 20.5 kcal∙mol−1 (1 cal = 4.1868 J); the coefficient b was very small and could be ignored. The quantity Δhs can be given as XΔhDPPC + Δhmix, where ΔhDPPC is the gel – liquid crystalline transition enthalpy of DPPC (8.74 kcal∙mol−1) and Δhmix is the enthalpy of mixing the two liquid crystalline lipids. Δhmix depends on X in approximately a parabolic fashion, having a maximal value of 4.8 kcal∙mol−1 at X = 0.6.It was shown that both the solubility and mixing enthalpy data can be described by the theory of regular solutions (RST). In RST, the activity coefficient of the solute (component 2) of a binary solution is given by RTlnγ2 = (1 − θ2)2ΔU, while the mixing enthalpy is given by Δhmix = θ1θ2 ΔU/v2, where θ1 and θ2 are the volume fractions of solvent and solute (egg PC and DPPC, respectively), v2 is the partial molar volume of DPPC, and ΔU is the energy change per mole on interchanging a DPPC and an egg PC molecule between their respective liquid crystalline phases. The thermodynamic data are accurately described by RST, the molar volume of DPPC being found to be about half mat of egg PC solution and the interchange energy ΔU having a value of 10–11 kcal∙mol−1. There was some evidence that ΔU may be an increasing function of temperature. The large value of the ΔU accounts for the pronounced temperature dependence of the solubility Xsat, which decreases from 0.35 at 35 °C to 0.02 at 10 °C.The presence of cholesterol in the mixtures decreases both the transition enthalpy of DPPC and the mixing enthalpy in a linear fashion, so that Δhs is zero at Xcholesterol ≥ 0.2. The results are consistent with recent data indicating the formation of a PC–cholesterol complex of stoichiometry approximately 4:1.