A combined experimental and theoretical study of phase behavior and structure in binary systems of block copolymers and a selective solvent is presented. The block copolymers used here consist of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), and the solvent is water, selective for the PEO block. The concentration-temperature phase diagrams of (EO)(8)(PO)(47)(EO)(8) (Pluronic L92) and (EO)(11)(PO)(70)(EO)(11) (Pluronic L122) in water have been determined experimentally. These two block copolymers are unique in that they form both normal (H-1, "oil-in-water") and reverse (H-2, "water-in-oil") hexagonal lyotropic liquid crystalline structures in binary systems with water over the same temperature range. This is the first time the H-2 structure is reported in a binary PEO-PPO-PEO/water system. Thereafter, the predicted phase diagram of (EO)(20)(PO)(69)(EO)(20) in water is given, employing a self-consistent mean-field lattice theory with internal degrees of freedom. The free energy for a number of possible microstructures was calculated as a function of the polymer concentration, and the extensions of the one-phase and two-phase regions were established. The model predicted the composition and temperature stability ranges of disordered solution, micellar solution, normal hexagonal, lamellar, reverse hexagonal, and reverse cubic ordered phases which appeared at increasing (EO)(20)(PO)(69)(EO)(20) concentration, in good agreement with the experimental Pluronic L122 phase behavior. Moreover, the model provides the volume fraction profiles for the PEO, PPO, and water components in the self-assembled microstructures, information which is not readily accessible from experiments. Finally, an increased segregation among the species and an increased domain spacing at increasing polymer length were found. The data are consistent with the scaling behavior predicted for the domain size in weakly segregating block copolymer systems.