Changing a glassy polyimide dianhydride-diamine bond location from para to meta has been shown experimentally to lead to significant decreases in gas permeabilities. This work is aimed at comparing a fully-atomistic molecular model of a mew-linked 6FDA-6FmDA polyimide membrane to its para-linked 6FDA-6FpDA isomer using large-scale molecular dynamics simulations (MD). The effects of structural isomerism on the early stages of CO2 sorption and matrix plasticization are also assessed. Both isomers are close in densities, but the meta-chains are more coiled than the para-chains, their void-space is slightly smaller and their mobility is lower. The higher resistance of the meta-matrix to localized chain motions leads to a better conservation of its cohesion and a reduced volume dilation upon CO2 entry into the membranes. In agreement with its larger dilating capacity, the gas uptake is always higher in the para isomer. The concave behavior of the sorption curves compares favorably with experimental data, except for the highest-pressure system which exhibits a quasi-supercritical behavior for CO2. However, in all cases, swelling is associated to local relaxations of the matrices rather than to large changes in the structures, while glassy chain mobility remains fairly restricted. Gas mobility is directly correlated to the underlying mobility of the matrices and, as such, is slower in the meta than in the para isomer. (C) 2014 Elsevier B.V. All rights reserved