The effects of supercritical CO2 (scCO(2)) conditioning are examined through changes in gas permeability and solubility for 6FDA-based polyimide membranes. At pressures above the supercritical point, both CO2 permeability and sorption unexpectedly decline, suggesting a structural rearrangement of the polymer facilitated by the presence of a highly soluble species. High-pressure permeability isotherms for He and N-2 do not exhibit a similar decline. Upon depressurization, a typical hysteresis is not observed; rather, CO2 permeability remains at reduced levels compared to the original pressurization, Permeability isotherms of the conditioned polymer show a reduction in transport compared to unconditioned samples; however, sorption isotherms show an increase in penetrant solubility following scCO(2) conditioning. Pre-and post-CO2/CH4 mixed gas permeation testing at 35 degrees C shows a reduction in membrane permeability with a corresponding increase in the separation factor after scCO(2) conditioning.