The phase behavior of a styrene-isoprene (SI) diblock copolymer, with block molecular weights of 1.1 x 10(4) and 2.1 x 104 g/mol, respectively, is examined in the neutral solvent bis(2-ethylhexyl) phthalate (DOP) smd the styrene-selective solvent di-n-butyl phthalate (DBP). DBP is a good solvent for PS, but is near a theta solvent for PI at approximately 90 degrees C. Small-angle X-ray scattering (SAXS), rheology, and static birefringence are used to locate and identify order-order (OOT) and order-disorder transitions (ODT); all three techniques gave consistent results. The neat polymer adopts the gyroid (G) phase at low temperatures, with an OOT to hexagonally-packed cylinders (C) at 185 degrees C, and the ODT at 238 degrees C. Upon dilution with the neutral solvent DOP, the C window is diminished, until for a polymer concentration phi = 0.65, a direct G to disorder (D) ODT is observed. These results reflect increased stability of the disordered state, based on the different concentration scalings of the interaction parameter, chi, at the OOT and ODT. The OOT follows the dilution approximation, i.e., chi(OOT) similar to phi(-1), but the ODT is found to follow a stronger concentration dependence, i.e., chi(ODT) similar to phi(-1.4), Similar to the scaling of phi(-1.6) found previously for lamellar SI diblocks in toluene and DOP. Addition of the selective solvent DBP produces dramatic changes in the phase behavior relative to DOP and the melt state; these include transitions to lamellar (L) and perforated layer (PL) structures. The observed phase sequences can be understood in terms of trajectories across the SI melt phase map (temperature vs. composition) : addition of a neutral solvent or increasing temperature corresponds to a "vertical" trajectory, whereas adding a selective solvent amounts to a "horizontal" trajectory. When the solvent selectivity depends on temperature, as it does for the SI/DBP system, increasing temperature results in a diagonal trajectory. For both neutral and selective solvents the domain spacing, d*, scales with phi and chi as anticipated by self-consistent mean-field theory.