A rich phase behavior with lamellar, normal hexagonal, reverse hexagonal, and bicontinuous cubic liquid crystalline regions, as well as water-rich (L(1)) and oil-rich (L(2)) solutions, was observed at 25 degrees C in a ternary system composed of water, p-xylene, and Pluronic 25R4, an amphiphilic block copolymer of ethylene oxide (EO) and propylene oxide (PO) with the structure (PO)(19)(EO)(33)(PO)(19). The phase boundaries were identified using ?H-NMR and inspection under polarized light. Small angle X-ray scattering was employed to ascertain the structure of the various Liquid crystalline phases formed and to determine the characteristic lengths involved. The sequence of L(1), normal hexagonal, lamellar, reverse hexagonal, and L(2) phases was observed when replacing water by oil at a constant polymer weight fraction of 0.5. The volume fraction of the apolar components (PPO + oil) can be identified as the major parameter governing the phase behavior. The ternary phase behavior of the PPO-PEO-PPO copolymer is similar to that of the opposite block sequence PEO-PPO-PEO copolymer (Pluronic L64) of comparable molecular weight to 25R4.