The separation of hydrocarbons from permanent gases is of considerable importance in the chemical industry. Poly(1-trimethylsilyl-1-propyne) [PTMSP] is extremely permeable to hydrocarbons and has high hydrocarbon/permanent gas selectivity. However, the poor chemical resistance of this material limits its use as a membrane for industrial applications. To overcome this problem, we studied an alternative acetylene-based polymer, poly(4-methyl-2-pentyne) [PMP], which exhibits much better chemical resistance than PTMSP Several types of non-porous, nanosized, fumed silica fillers were incorporated in PMP to manipulate the molecular polymer chain packing. The pure-and mixed-gas permeation properties of the PMP/silica hybrid membranes were studied. The gas permeability and the hydrocarbon/permanent-gas selectivity increased simultaneously with increasing filler content. The n-butane/methane selectivity was 13 for pure PMP, but increased to 26 for 45 wt% silica-filled PMP. In addition, the n-butane permeability also increased 3-4 fold. Therefore, the silica-filled hybrid PMP membrane showed completely opposite gas permeation behavior to that of conventional polymers filled with non-porous inorganic nanoparticles.