The transport properties of a series of fluorinated polyimide-organosilicate hybrid membranes were characterized as a function of feed pressure to determine the degree Of CO2 plasticization. The pure gas feed pressures ranged from 4 to 30 atm using CO2 and CH4 at 35 degreesC. The sol-gel derived organosilicate domains were formed using partially hydrolyzed alkoxide precursors, which were cross-linked with the polymer matrix. The degree of plasticization was dependent upon the degree of cross-linking, being the lowest in a highly cross-linked hybrid membrane. Separate analysis of the diffusion and sorption behavior of CO2 and CH4 suggested that the plasticization phenomenon results more from changes in local chain dynamics rather than increases in free volume due to swelling. (C) 2003 Elsevier B.V. All rights reserved.