The phase behavior of PPO (sn-1,2-dipalmitoyl-sn-3-oleoylglycerol) and POP (sn-1,3-dipalmitoyl-sn-2-oleoylglycerol) binary mixture was examined using differential scanning calorimetry, conventional and synchrotron radiation X-ray diffraction (XRD), and highly pure samples. A molecular compound, beta(c), was formed at the 1:1 concentration ratio of PPO and POP, giving rise to two monotectic phases of PPO/compound and compound/POP in a juxtapositional way. beta(c) has a long spacing value of 4.1 nm of double chain length structure. In the PPO/compound region, the DSC melting peak increased with increasing PPO concentration, whereas the DSC melting peak increased with increasing POP concentration in the compound/POP region. The melting point of beta(c) was lowest at 31.2 degrees C. Time-resolved XRD study unveiled the formation of molecular compounds in metastable forms, alpha(c) and beta’(c), having the same PPO/POP concentration ratio as beta(c). alpha(c) and beta’(c) exhibited monotectic phases with corresponding metastable forms of the pure components. In alpha(c), two hexagonal packing XRD short spacing peaks were obtained for alpha c, corresponding to a differently packed hexagonal subcell of two leaflets. A structure model of the PPO/POP molecular compound is proposed, involving separation of palmitoyl chain leaflet and palmitoyl-oleoyl mixed-acid chain leaflet in the double chain length structure. This model justifies the structure of beta(c) of SOS/SSO proposed by Engstrom (Engstrom, L. J. Fat Sci. Technol. 1992 94, 173-181), in which the saturated and unsaturated chains are packed in the same leaflet of the double chain length. The present work additionally proved that the formation of the molecular compound was present in the alpha(c) and beta’(c) metastable forms, as directly proved by dynamic X-ray diffraction study using synchrotron radiation.