A room-temperature chemical cross-linking modification has been performed on Matrimid((R)) 5218, a material that has received pervasive attention in membrane separation. The cross-linking reaction was conducted by simply immersing the membrane in a p-xylenediamine solution at ambient temperature for a stipulated period of time. The influence of cross-linking modification on thermal and gas transport properties of Matrimid membrane were studied. The gas permeability and selectivity of pure gases He, O-2, N-2, CH4 and CO2 as well as mixed gas of CO2/CH4 were measured. The gas permeabilities show their maximum values for 1-day cross-linked membranes, but then decrease gradually with immersion time. It is found that the Matrimid membrane undergoes the plasticization phenomenon at 15 atm, however it is effectively suppressed by the proposed cross-linking modification. The ideal selectivities for O-2/N-2, CO2/CH4 and CO2/N-2 remain almost constant and decreased slightly with the cross-linking reaction, whereas the He/N-2 selectivity increased with immersion time. Thus, it appears that the cross-linking modification on Matrimid is mainly useful for separation of gas He/N-2. The mixed gas permeabilities for both CO2 and CH4 are lower than their pure gas permeabilities at respective partial pressure. While the mixed gas selectivity is found to be lower for untreated membranes but higher for cross-linked membranes, if we compared with the ideal selectivity of CO2/CH4. (C) 2003 Published by Elsevier B.V.