Poly(4-methyl-2-pentyne) (PMP) has been crosslinked using 4,4'-(hexafluoroisopropylidene) diphenyl azide (HFBAA) to improve its chemical and physical stability over time. Crosslinking PMP renders it insoluble in good solvents for the uncrosslinked polymer. Gas permeability and fractional free volume (FFV) decreased as crosslinker content increased, while gas sorption was unaffected by crosslinking. Therefore, the reduction in permeability upon crosslinking PMP was due to decrease in diffusion coefficient. Compared to the pure PMP membrane, the permeability of the crosslinked membrane is initially reduced for all gases tested due to the crosslinking. By adding nanoparticles (FS, TiO2), the permeability is again increased; permeability reductions due to crosslinking could be offset by adding nanoparticles to the membranes. Increased selectivity is documented for the gas pairs O-2/N-2, H-2/N-2, CO2/N-2, CO2/CH4 and H-2/CH4 using crosslinking and addition of nanoparticles. Crosslinking is successful in maintaining the permeability and selectivity of PMP membranes and PMP/filler nanocomposites over time. (c) 2008 Elsevier B.V. All rights reserved.