The phase-separated morphology of three different molecular weights of polystyrene-block-polyisoprene (PS-b-PI) swollen with mixed solvents was studied by small-angle X-ray scattering (SAXS) and UVvis spectroscopy. The solvent mixtures consisted of dimethyl phthalate (DMP) and n-tetradecane (C14), selective solvents for PS and PI, respectively, and a neutral solvent, di-n-octyl phthalate (DOP). DMP and C14 are immiscible and hence selectively sequestered in the corresponding phases. SAXS measurements confirmed that a lamellar morphology was observed in most cases and that the domain spacing depended on the polymer molar mass, the concentration in solution, and the composition of the mixed solvent. The incompatibility of the PS and PI phases was increased by addition of DMP and C14 and decreased by addition of the neutral solvent DOP. The domain spacing was varied in this manner over a wide range. The diblock copolymer PS-b-PI with the largest molecular weight showed reflection peaks at wavelengths from 345 to 793 nm. The results are supported by calculations of the structure of the block copolymers in the presence of the solvents on the basis of the self-consistent field theory.