Poly(1-trimethylsilyl-1-propyne) (PTMSP), the most permeable polymer known, exhibits an unusual dependence of permeability on temperature. Permeability of most gases in PTMSP decrease with increasing temperature, which is in contrast to the behavior of polysulfone and other glassy polymeric membranes. Cavity size (free volume) distributions of PTMSP are calculated at different temperatures using an energetic-based cavity-sizing algorithm. The distributions of cavity sizes in PTMSP are shifted to smaller cavities at higher temperature. The diffusion, solubility and permeation of gases in PTMSP were also simulated at different temperatures. Good agreement was obtained between experimental data and simulation results. The smaller cavity size in PTMSP at higher temperature may contribute to the lower permeability of gases in PTMSP at higher temperature. (C) 2008 Elsevier B.V. All rights reserved.