The work presents an approach to reduce the plasticization of polymeric membranes caused by condensable gases, and particularly the effect of plasticization caused on polyimides by CO2 at high pressure. A technical polyimide, Matrimid(R), was chosen as a reference of polyimide membrane and the approach applied consisted of incorporating reactive oligomers to have cross-linkable mixed systems, which do not plasticize at high CO2 pressure. Films of semi-interpenetrating networks (semi-IPNs) based on Matrimid(R) and phenolphthalein dicyanate as cross-linking monomer in ratios 90/10, 80/20, and 70/30, were prepared using a catalyst to lower the curing temperature from 280 to 180 degrees C. Semi-IPNs properties such as thermal stability, mechanical properties, glass transition temperatures, or density were measured to characterize the films and were correlated with the dicyanate monomer content. The CO2 gas permeation behavior of the three semi-IPNs was studied using a CO2 feed pressure ranging from 1 to 30 atm. The gas separation properties were mainly explained attending to the density of the films, which depended on the dicyanate content used. In the three catalyzed semi-IPNs, the CO2 permeability coefficients remained almost constant all along the investigated range of CO2 pressure while Matrimid(R) treated at 180 degrees C did show a clear tendency to plasticization over a critical feed pressure of about 17 bar. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012