In this study, we have semisynthesized the following three molecular species of mixed-chain phosphatidylethanolamine: C(22):C(12)PE, C(16):C(18:1 delta 9)PE, and C(10):C(24:1 delta 15)PE. These lipids share a common structural characteristic, that is, they all have the same total number of carbon atoms in their acyl chains. Aqueous dispersions prepared from three sets of binary lipid mixtures, C(16):C(18:1 delta 9)PE/C(22):C(12)PE, C(10):C(24:1 delta 15)PE/C(22):C(12)PE, and C(16):C(18:1 delta 9)PE/C(10):C(24:1 delta 15)PE, were studied by high-resolution differential scanning calorimetry, leading to the construction of three temperature-composition phase diagrams. A computer program developed on the basis of the thermodynamic equations for non-ideality of mixing (or Brigg-Williams approximation) was applied to fit the calorimetric data, yielding the non-ideality parameters of mixing in the gel and the liquid-crystalline bilayers (pG and pL). Based on the shapes of these phase diagrams and the values of pG and pL, it is concluded that any two of the three molecular species of phosphatidylethanolamines under study can mix nearly ideally in the bilayer plane of the liquid-crystalline bilayer. However, these binary lipid mixtures do exhibit the gel-gel phase immiscibility over an extensive compositional region in the gel-state bilayer. By comparison with experimental data obtained with binary mixtures of saturated identical-chain phospholipids, we can conclude that mixed-chain cis-monounsaturated lipid molecules and saturated lipid molecules are highly demixed in the same two-dimensional plane of the gel-state bilayer, although the bilayer thickness difference between the lipid bilayer composed of cis-monounsaturated lipids and that of saturated lipids may be only one or two C-C bond lengths at T < Tm.