Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) has been crosslinked using 3,3'-diazidodiphenylsulfone to improve its solvent resistance and physical stability. This study reports the influence of crosslinking on N-2, O-2 and CH4 gas permeabilities and fractional free volume (FFV) as a function of time. Crosslinking PTMSP renders it insoluble even in excellent solvents for the uncrosslinked polymer. The gas permeability and FFV of uncrosslinked and crosslinked PTMSP decreased over time, so crosslinking PTMSP does not arrest physical aging. The addition of 10 wt.% polysiloxysilsesquioxanes (POSS) nanoparticles decreased the permeability of PTMSP by 55%, and the permeability and FFV values were stable over time for PTMSP films containing 10 wt.% POSS nanoparticles. The permeability of PTMSP at a given FFV was greater than that of other substituted polyacetylenes, polysulfones or polycarbonates, which is consistent with differences in the arrangement of free volume in these polymers, as probed by positron annihilation lifetime spectroscopy (PALS). Ellipsometry was used to characterize physical aging of thin (similar to 400 nm) uncrosslinked and crosslinked PTMSP films supported on silicon wafers. The ellipsometry results showed that crosslinking does not markedly slow physical aging of thin PTMSP films. Published by Elsevier B.V.