This work reports the CO2 plasticization of gas-separation hollow-fiber membranes based on polyimide and polyethersulfone blends. The feed pressure effect on the permeance of pure gases (CO2, N2) and the separation performance of a gaseous mixture (CO2/N2, 55/45%) is examined. Contrary to dense membranes, the permeance of CO2 through ultrathin asymmetric fibers increases immediately with pressure resulting in pronounced apparent plasticization and reduction of the ideal CO2/N2 selectivity. However, no evidence of plasticization was observed when a CO2/N2, 55/45% mixture was fed to the hollow-fiber membranes. In all cases, CO2 permeance decreased with pressure, while that of N2 remained constant. Experimental results were validated by means of mathematical modeling. Membrane-separation performance was overestimated when pressure-independent permeabilities were used in the model, while pressure-dependent permeabilities, due to the overall effect of plasticization and competition phenomena, explained excellently, the obtained stage-cut and permeate purity.