Interdomain spacing, d, of microphase-separated poly(styrene-b-dimethylsiloxane) in high-pressure carbon dioxide (CO2) was found to follow a modified power law scaling, d/xi similar to phi(-gamma)(diblock), where xi is a swelling asymmetry factor and phi(diblock) is the volume fraction polymer. This interdomain spacing was measured in situ both isothermally and at constant phi(diblock) using small-angle neutron scattering. The use of high-pressure CO2 as the diluent enabled precise control of sorbed solvent volume fraction in highly concentrated systems at elevated temperatures. Data measured at constant phi(diblock) are in agreement with theoretical predictions and experimental data for dilution by traditional liquid solvents and follow the scaling law d similar to chi(-beta)(AB). In the highly concentrated regime, the isothermal dependence on phi(diblock) cannot be described by the simple power law scaling previously suggested for liquid diluents, d similar to phi(-gamma)(polymer). By accounting for solvent selectivity using xi, however, d-spacing as a function of phi(diblock) for a system exhibiting lamellar morphology collapses to a common scaling.