The phase behavior and microstructure at 25 degrees C in mixtures of the amphiphilic block copolymer Pluronic P104 (with the formula (EO)(27)(PO)(61)(EO)(27), where EO is ethylene oxide and PO is propylene oxide), D2O, and p-xylene are presented. A rich phase behavior with both normal ("oil-in-water") and reverse ("water-in-oil") phases is observed. Two isotropic micellar solutions (normal and reverse micellar solutions) and six lyotropic liquid crystalline phases (normal and reverse micellar cubic, normal and reverse hexagonal, reverse bicontinuous cubic, and lamellar phases) are formed. The structural length scales in the liquid crystalline phases were determined from small-angle X-ray scattering. The interfacial area per PEO block varies in the range 110-155 Angstrom(2).