Synthesis of polyetherimide-bismaleimide (PEI-BMI) semi-interpenetrating polymer networks (semi-IPNs) combined with solvent phase inversion was used to prepare asymmetric flat membranes, which were coated with silicon rubber. These membranes were evaluated for production of O-2-enriched air and separation of CO2 from its mixture with CH4 as well as with N-2. Using different preparation schemes membranes with varying skin and supported layer characteristics were prepared. These different morphologies of membranes were responsible for a trade-off performance between gas permeance and perm selectivity. These new composite, PEI-BMI semi-IPN membranes showed suitable performance for production of O-2-enriched air and separation Of CO2 from natural gas or flue gas relevant to greenhouse gas emission control. Membrane performance was explained in terms of the intrinsic gas transport properties of the coated silicon layer and membrane glassy material, which determine the limitations of permeance-permselectivity trade-off. It was also found that the permselectivity for CO2 over CH4 or N-2 increases with increasing CO2 feed concentration. This might be exploited to arrange for more cost-efficient multistage gas separation systems. Crown Copyright (c) 2006 Published by Elsevier B.V. All rights reserved.