The effect of the thermodynamic selectivity of a solvent on the self-assembly of a styrene-isoprene (SI) diblock copolymer with block molecular weights of 1.1 × 104 and 2.1 × 104 g/mol is investigated. We explore the phase behavior from the melt state to dilute solution in solvents that are of varying selectivities for the two blocks. Bis(2-ethylhexyl) phthalate (DOP) is a neutral good solvent for SI. Di-n-butyl phthalate (DBP) and diethyl phthalate (DEP) are good solvents for PS but marginal and poor, respectively, for PI. Tetradecane (C14) is utilized as a complementary solvent that is good for PI but poor for PS. Small-angle X-ray scattering (SAXS) and static birefringence are used to locate and identify order-order (OOT) and order-disorder transitions (ODT). Dynamic and static light scattering were used to characterize the dilute solution micellar behavior. The neat polymer forms the gyroid (G1) morphology at low temperature, an OOT to hexagonal-packed cylinders (C1) occurs at 185°C, and the ODT is located at 238°C. Dilution with DOP decreases the OOT temperature in agreement with the dilution approximation, i.e., χOOT approx. φ-1, but the ODT follows a stronger dependence of χODT approx. φ-1.4. The slightly selective solvent DBP stabilizes the ordered state relative to DOP. Rich lyotropic and thermotropic behavior is observed among regions of lamellae (L), inverted gyroid (G2), inverted hexagonal-packed cylinders (C2), and inverted body-centered-cubic spheres (S2bcc). Solutions in DEP display similar morphological behavior, along with significantly increased ODT temperatures. Because of the asymmetric block copolymer composition, the phase behavior in the isoprene-selective solvent C14 is markedly different, as only G1, C1, and S1bcc phases are observed. The overall sequence of phases with dilution and/or heating is rationalized on the basis of diagonal trajectories across the phase map (temperature vs composition) for undiluted block copolymers: addition of a neutral solvent corresponds to increasing the temperature and thus a 'vertical' trajectory, whereas the partitioning between microdomains of a selective solvent amounts of a 'horizontal' trajectory to a renormalized block volume composition. However, a variety of novel features are observed in DEP: the formation of face-centered-cubic packed micelles, a reentrant thermotropic ODT, and a large window of L + C2 coexistence. The dependence of the principal length scale, d*, on φ, T, and structure is also considered. The strongly temperature-dependent selectivity produces a crossover in the scaling of d* vs φ for the lamellar phase: the addition of a selective solvent increases d*, but as the solvent becomes neutral, d* decreases. This phenomenon is captured by self-consistent mean-field calculations.