The phase behavior and structure of binary amphiphilic polymer−water systems have been studied as a function of polymer concentration and temperature for three poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) copolymers of different composition, (EO)6(PO)34(EO)6 (L62), (EO)13(PO)30(EO)13 (L64), and (EO)37(PO)58(EO)37 (P105), by using 2H-NMR and small-angle X-ray scattering (SAXS). The number of lyotropic liquid crystalline (LLC) phases formed increases with the poly(ethylene oxide) content and the molecular weight of the polymers in the order L62 < L64 < P105. Only a lamellar LLC phase is formed by L62, while P105 is capable of self-assembling with increasing polymer concentration into (body-centered close-packed) cubic, hexagonal, and lamellar LLC phases. Upon heating, the LLC phases of the L62−water and L64−water systems swell with water; no such swelling is detected for the P105−water system. The thermal stability of the LLC regions increases in the order cubic < hexago...