The phase diagram of fully hydrated binary mixtures of dipalmitoylphosphatidylcholine (DPPC) with 1,2-dipalmitoylglycerol (DPG) published recently by López-García et al. identifies regions where stoichiometric complexes of 1:1 and 1:2 DPPC:DPG, respectively, are formed. In this study, the structural parameters of the 1:1 complex in the presence of pure DPPC was characterized by synchrotron low angle and static x-ray diffraction methods. Structural changes upon transitions through phase boundaries were correlated with enthalpy changes observed by differential scanning calorimetry in mixtures of DPPC with 5, 7.5, 10, and 20 mol% DPG dispersed in excess water. Phase separation of a complex in gel phase could be detected by calorimetry in the mixture containing 5 mol% DPG but was not detectable by synchrotron low angle x-ray diffraction. Static x-ray measurements show evidence of phase separation, particularly in the reflections indexing chain packing. In the mixture containing 7.5 mol% DPG, two distinct lamellar repeat spacings could be seen in the temperature range from 25 to 34 degrees C. The lamellar spacing of about 6.6 nm was assigned to pure gel phase DPPC because the change in the spacing corresponds with thermal transition of the pure phospholipid, and a longer repeat spacing of about 7.2 nm was assigned to domains of the 1:1 complex of DPPC-DPG. In the temperature range from 34 to 420C, i.e., in the region of coexistence of the ripple phase of DPPC and the gel phase of the complex, a single, rather broad lamellar reflection appears because of superposition of two reflections of DPPC and the complex; the lamellar spacing of DPPC in the ripple phase is similar to that of the gel phase of complex. In the coexistence region of the liquid-crystalline phase of DPPC and the gel phase of complex (-42-480C), the lamellar reflections of the both phases are present. The fluidus boundary lies between the coexistence region and the fluid region.In the fluid region (-48-550C), the gel state of complex persists up to the fluidus boundary, whereupon the liquid-crystalline state of complex replaces the gel state of the complex. This indicates that the complex is also immiscible with DPPC even above the fluidus boundary at least in the temperature range close to the phase boundary. For mixtures comprising 10 and 20 mol%DPG in DPPC, complex formation is clearly detectable in both the gel region and the coexistence region by x-ray diffraction.Synchrotron x-ray measurements indicate phase separation between pure DPPC and liquid-crystalline complex just above thefluidus boundary. Static, wide angle x-ray measurements also suggest phase separations of the 1:1 complex not only from the gel phase but also the liquid-crystalline phase of pure DPPC. Two distinct diffraction peaks were detected for the mixture of DPPC with 5, 10, and 20 mol% DPG. One is due to the chain spacing of the complex, and the other is due to that of the pure DPPC. In the coexistence region of the liquid-crystalline phase of DPPC and the gel phase of complex, two kinds of diffraction peaks of the hydrocarbon chain of the gel phase complex and the broad scattering profile for the chain melting of DPPC were observed in the wide angle region. Electron density reconstructed from the lamellar reflections indicates that the thicknesses of both the bilayer and the water layer of the gel phase complex are greater than those of the respective thicknesses of gel phase DPPC.