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 H-2-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 < hexagonal < lamellar and L62 < L64 < P105. An increase of the temperature results in a decrease in the interfacial area and an increase in the periodicity of the L62 and L64 lamellae. In the P105-water system, the structural dimensions in the lamellar and hexagonal LLC regions are not much affected by temperature. Both the lamellar periodicity and the block copolymer interfacial area decrease with increasing polymer content, for all polymers. The factors influencing the self-assembly mode of amphiphilic copolymers are discussed, and the phase behavior of the PEO-PPO-PEO copolymers in water is compared to that of nonionic surfactants.