The self-assembly behavior in water,of a mixture of two poly (ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymers, (EO)(13)(PO)(30)(EO)(13) (L64) and (EO)(37)(PO)(58)(EO)(37) (P105), was explored at 25 degrees C. The phase boundaries were established using H-2-NMR and inspection under polarized light; the structure of the various lyotropic liquid crystalline (LLC) phases was determined with small-angle X-ray scattering, while viscosity and differential scanning calorimetry measurements were used to probe the isotropic water-rich solution region. Isotropic regions, similar to the neat polymers, are stable at high polymer content. The addition of water induces structure in the amphiphilic block copolymer system. An extended lamellar (D)LLC phase is formed at 20-25% water content; a hexagonal (E) and a cubic (I) LLC phases supersede D at higher water contents. In addition to the above, a narrow isotropic region (L’) is observed on the L64-water binary axis, in equilibrium with the E and the D phases. The hexagonal and lamellar LLC phases extended all the way from the L64-rich to the P105-rich side of the ternary L64-P105-water phase diagram; the characteristic hexagonal and lamellar structural dimensions varied linearly with P105 content in the L64-P105 mixture at a constant water concentration, An isotropic (micellar) solution phase (L(1)) dominates the high-water content corner of the ternary phase diagram, Viscosity measurements in this region provided evidence for increased interactions between the micelles as the boundary to the LLC phases was approached.