Composite membranes with a rubbery polymer skin layer are promising for the separation of organic vapours (e.g., light olefins and volatile organic compounds (VOCs)) from nitrogen for emission control and recovery of valuable components. Because of the high flux of composite membranes, the resistance of the substrate becomes increasingly important to the overall permeation. The effect of substrate resistance on gas permeation was studied using flat poly(ether block amide)/polysulfone (PEBA/PSf) and poly(dimethyl siloxane)/polyetherimide (PDMS/PEI) hollow fiber composite membranes. It was found that for a given substrate, the selectivity of the PEBA/PSf membrane for the separation of binary methanol/N-2 and ethanol/N-2 mixtures decreased as the PEBA skin layer thickness was reduced. The selectivity of the PDMS/PEI composite membrane to C2H4/N-2 and C3H6/N-2 was much lower than the intrinsic selectivity of PDMS. This is attributed to the substrate resistance that represents a significant contribution to the overall mass transport resistance for the permeation of highly permeable components (e.g., VOCs), while the skin layer may still dominate the permeation of slow permeant (e.g., N-2). Depending on the magnitude of the substrate resistance relative to the skin layer, the membrane permselectivity can be compromised substantially. In the development of advanced composite membranes, when the skin thickness is reduced, the substrate structure should be optimized so as to minimize the effect of the substrate resistance. (C) 2009 Elsevier B.V. All rights reserved.