Nanocomposite polymer and ultrathin film membranes have shown great promise in enhancing gas permeation and selectivity properties by interfacially straining polymer matrices, yielding structures of higher free volume. However, undesired particle aggregation and short temporal stability remain a big challenge. In the present study, an "inverse" architecture to conventional polymer nanocomposites was investigated, in which the polymer phase poly(1-trimethylsilyl-1-propyne) (PTMSP) was interfacially and dimensionally constrained in nanoporous anodic aluminum oxide (AAO) membranes. While with this architecture the benefits of nanocomposite and ultrathin film membranes could be reproduced and improved upon, also the temporal stability could be enhanced substantially. Gas permeabilities of helium, nitrogen and carbon dioxide were increased over five-fold, and selectivities of CO2/He and CO2/N-2 could be enhanced by 40% compared to the pristine bulk phase, while physical aging, caused by free-volume collapse, was reduced twenty-fold compared to ultrathin membranes. (C) 2013 Elsevier Ltd. All rights reserved.