The phase behavior of a monoolein-rich (89.5 wt %) monoglyceride blend based on pine needle oil (denoted glycerol monooleate)-poloxamer 407 block copolymer-water system has been investigated and studied by small-angle X-ray diffraction. The phase diagram exhibits an extended cubic phase field, originating from the binary glycerol monooleate-water system, ranging from 18 to 67 wt % water with a maximum content of 20 wt % of poloxamer 407. It is shown to possess four different reversed bilayer-based cubic one-phase regions as based on phase equilibria and X-ray data. X-ray data indicates that these are of crystallographic cubic aspects number 4, 8 (two cubic structures are proposed to have the same symmetry), and 12. The structures are further identified by reference to the minimal surface bisecting the three-dimensional bilayer in each structure. The cubic phase sequence is proposed to be G(Q(230)) --> D(Q(224)) --> P(Q(229)) --> C(P)(Q(229)), described by Schoen’s gyroid, Schwarz’ double diamond, Schwarz’ primitive, and Neovius’ periodic minimal surfaces, respectively. The lattice parameters vary approximately between 130 and 150 Angstrom, 75 and 102 Angstrom, 120 and 210 Angstrom, and 315 and 355 Angstrom for these phases, respectively, and are found to be most influenced by the proportion of water. At concentrations higher than 20 wt % poloxamer 407 the cubic phase P(Q(229)) is transformed to a reversed micellar (L(2)) phase. The phase diagram possesses two lamellar (L(alpha)) phases, one which originates from the binary glycerol monooleate-water system which extends to about 15 wt % of poloxamer 407. The other is located in the water-rich region at 65-75 wt % of water, and approximately equal amounts of poloxamer 407 and glycerol monooleate. At higher dilutions (about 80 wt % water) an isotropic flow-birefringent L(3) or sponge phase appears with domain sizes above 200 Angstrom. The fine structure of the different phases is discussed, with emphasis on the structure of the cubic phases, in terms of the mixing behavior and the molecular arrangement of the poloxamer 407 molecules and the glycerol monooleate-based bilayer.