A caulking layer is frequently applied to gas separation membranes to alleviate the effects of structural defects. Caulking materials have high gas diffusivities, so as not to hinder the performance of the gas separation layer. Despite these considerations, for some bi-layer systems unaffected by plasticization, the mixed gas selectivity has been observed to be significantly lower than selectivity values calculated from pure gas permeabilities. The present object is to model and simulate two-layer caulked gas separation membranes at the fundamental level to discover the mechanistic basis for the observed behaviour. To this end, a transport based model considering intrinsic material properties was developed to simulate gas separation for a system of CO(2) and N(2) diffusing across a caulking layer of polydimethyl siloxane (PDMS) and then a polyetherimide-polyethylene glycol interpenetrating network (PEI-PEG IPN) membrane. The model was able to account for flux declines in mixed gas systems greater than those predicted by conventional series resistance models, such as have been observed experimentally. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.