The phase behavior of nearly symmetric styrene-isoprene (SI) diblock copolymers in the presence of compressed CO2 or ethane was studied using small-angle neutron scattering. Sorption of either fluid depresses the upper order-disorder transition (UODT) by as much as 55 degrees C at fluid densities of less than 0.35 g/cm(3). The solvent-induced transitions are fully reversible upon fluid desorption. The magnitude of the depression can be controlled by manipulating the mass fraction of fluid sorbed through pressure-mediated adjustments in solvent density. Consequently, the location of the UODT can be tuned over broad ranges, and the state of segregation can be controlled via isothermal adjustments in total system pressure. The influence of hydrostatic pressure, inherent to the use of compressed solvents, is small in comparison to the solvent effect over the range of pressures studied.