The study addresses the effects of block composition on the self-assembly (and resulting microstructure) of amphiphilic block copolymers in the presence of selective solvents ("water" and "oil") by examining the ternary phase behavior and structure of two copolymers, E20P70E20 and E100P70E100, having the same block architecture (ExPyEx) and P:poly(propylene oxide) middle-block size but different E:poly(ethylene oxide) end-block sizes (Pluronic P123, E20P70E20, contains 30% E and Pluronic F127, E100P70E100, 70% E). A characterization (using SAXS and deuterium NMR) of the ternary isothermal (25 degrees C) E20P70E20-butyl acetate-water and E20P70E20-butanol-water systems is presented first. A variety of lyotropic liquid-crystalline (LLC) phases are thermodynamically stable in the former (butyl acetate) system, both of the "normal" (oil-in-water) and of the "reverse" (water-in-oil) morphology. In the latter (butanol) system, the reverse LLC phases are not stable but are replaced by an extensive water-lean solution region under the influence of the amphiphilic character of butanol. Following the presentation of the E20P70E20-"oil"-water systems, the microstructure afforded by E20P70E20 is compared to that of E100P70E100 (Holmqvist, P.; Alexandridis, P.; Lindman, B. Macromolecules 1997, 30, 6788). In the E20P70E20 phase diagrams, the lamellar structure (of zero interfacial curvature) is the most extensive. The high-E (hydrophilic) content of E100P70E100, however, favors oil-in-water LLC structures with high interfacial curvature. The copolymer-water side of the E100P70E100 ternary phase diagrams is dominated by the micellar cubic LLC structure; for cylindrical and lamellar structures to form, significant amounts of oil are needed. No reverse LLC phases are formed by E100P70E100 in the presence of water and either butyl acetate or butanol. The normal hexagonal phase in the E100P70E100 systems occurs in approximately the same composition range as the lamellar phase in the E20P70E20 systems.