We have studied the phase behavior of the poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymer in a mixture of two miscible solvents: water and tetrahydrofuran (THF). The techniques used to examine the different polymers, structures, and phases formed in mixed solvents were static and dynamic light scattering, small-angle neutron scattering, nuclear magnetic resonance, and fluorescence microscopy. By lowering the water/THF mixing ratio X, the sequence unimers-micron-sized droplets-polymeric micelles was observed. The transition between unimers and the micron-sized droplets occurred at X similar to 0.75, whereas the microstructuration into core-shell polymeric micelles was effective below X similar to 0.4. At intermediate mixing ratios, a coexistence between the micron-sized droplets and the polymeric micelles was observed. Combining the different aforementioned techniques, it was concluded that the droplet dispersion resulted from a solvent partitioning that was induced by the hydrophobic blocks. Comparison of poly(n-butyl acrylate) homopolymers and poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymers suggested that the droplets were rich in THF and concentrated in copolymers and that they were stabilized by the hydrophilic poly(acrylic acid) moieties.