The phase behavior of eight poly(styrene-b-isoprene) diblock copolymers in the neutral solvent dioctyl phthalate (DOP) has been examined over the full concentration (ø) range and from room temperature up to 250 °C. The copolymer compositions (f) ranged from 0.15 to 0.76, and molecular weights were chosen to make the melt order - disorder transition (ODT) accessible, or nearly so. Definitive phase assignments were made on the basis of small-angle X-ray scattering (SAXS). Individual order - order transitions (OOTs) and ODTs were located by a combination of rheology, static birefringence, and SAXS; all three techniques gave equivalent results. The transition temperatures were converted to values of the effective interaction parameter, Χ, using an established relation for this system. For all compositions, ΧODT scales as ø-α, with α varying from 1.3 to 1.6. These results are in conflict with the mean-field dilution approximation (α = 1.0), as was previously demonstrated for lamellae-forming samples. This result is attributed to the additional fluctuation stabilization of the disordered state in two-component solutions. Three of the samples displayed OOTs: cylinder to sphere (f = 0.15), gyroid to cylinder (f = 0.31), and lamellae to gyroid (f = 0.33). In each case, ΧOOT scaled as ø-1.0, in agreement with the dilution approximation. This result underscores the reliability of mean-field theory for computing relative free energies of the various ordered phases. The domain spacing, d, scales approximately as ø0.33 and Χ0.25, independent of morphology, in good agreement with both mean-field theory and previous experiments.