The evolution in the self-assembly of an amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer in ternary systems with water and organic solvents of varying relative polarity is examined. The phase behavior and microstructure of two ternary systems consisting of Pluronic F127 (Poloxamer 407), water, and the pharmaceutically acceptable solvents propylene carbonate (4-methyl-1,3-dioxolan-2-one) or triacetin (glycerol triacetate) are presented. The microstructure of the lyotropic liquid crystalline phases formed and their characteristic length scales are determined from small-angle X-ray scattering (SAXS). The trends in the SAXS lattice spacing and the interfacial area per block copolymer molecule help establish the location of the solvents in the microstructure. The phase behavior of the two systems studied here is discussed in the context of ternary systems of Pluronic F127 with water and polar solvents ("glycols", e.g., propylene glycol) or apolar organic solvents ("oils", e.g., xylene). Propylene carbonate and triacetin are shown to have intermediate behavior between that of glycols and oils. The block copolymer structural polymorphism is modulated by the solvent preference to locate in different domains of the block copolymer microstructure.