As the applications for polymeric membranes expand, new challenges arise. One of the largest of these challenges is the plasticization caused by strongly swelling penetrants such as carbon dioxide at elevated pressures. A considerable amount of material research has investigated crosslinking of dense film membranes to increase plasticization resistance. This paper extends such materials research to include more practically relevant asymmetric hollow fibers. Crosslinkable polyimide fibers were spun and an ester crosslinking reaction was studied using chemical and spectroscopic techniques to characterize the extent of crosslinking and to relate the effect of the reaction on fiber stability. CO2 permeance and CO2/CH4 selectivity were studied at a variety of pressures and temperatures over time to yield indications of real-world separation performance. (c) 2005 Elsevier Ltd. All rights reserved.