Gas separation membranes with enhanced performance were developed by the introduction of nanosized magnesium oxide particles followed by treatment with silver ions. Firstly, nanocomposite membranes were fabricated by incorporating nanoscale magnesium oxide particles with different loadings into the Matrimid matrix. The addition of MgO nanoparticles led to an increase in gas permeability of membranes; the highest permeability was observed for the membranes containing 40 wt% MgO loading. However, the selectivity of nanocomposite membranes was less than the neat Matrimid. These changes can be mainly ascribed to the effect of pore dimensions of MgO nanoparticles which are larger than the size range of gas molecules. Heat treatment at temperatures below polymer glass transition temperature resulted in densified structure, while heat treatment above T-g resulted in further increase in gas permeability of membranes through the changes in free volume of membrane. Secondly, nanocomposite membranes with 20 wt% MgO were modified by silver treatment for stipulated periods of time. A dipping procedure was adopted for this process which resulted in the impregnation of silver ions into the nanocomposite membranes. It was found that the major driving force for penetration of silver ions was offered by MgO nanoparticles and polymer played the role of a carrier. The adsorption process was confirmed by different techniques and oxygen sites at the surface of MgO were recognized to be responsible for this process. The longer immersion time led to more silver adsorption into the membrane. All the modified membranes exhibited enhanced gas separation performance for selected gas pairs compared to the neat Matrimid membrane. The best performance was observed for nanocomposite membranes (with 20 wt% MgO) after a 10-day silver treatment in which the CO2/CH4 and H-2/N-2 selectivity increased by 50% and 35%, respectively. (c) 2007 Elsevier B.V. All rights reserved.