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The relationship between poly(1-trimethylgermyl-1-propyne) (PTMGP) microstructure and gas permeability property is reported. Controlling the synthesis conditions via TaCl5 and NbCl5 catalyst systems, tailor-made polyacetylenes in a wide range of cis/trans ratio (trans content from 35 to ∼100%) were obtained. According to the cis and trans ratio in the polymeric chain, a two-phase structure was found characterized by the presence of regions with enhanced level of macrochains ordering and with amorphous regions. The polymer microstructure has been related with free-volume, gas permeability, and polymer resistance toward liquid hydrocarbons. PTMGP membranes with high trans content (80−90%) exhibited higher gas permeability than samples with low trans content. Gas permeability of PTMGP membranes decreased with the density of polymer films, i.e., with the free-volume fraction in polymers with different microstructures. Furthermore, it was demonstrated that gas permeability and resistance toward solvents of the PTMGPs are directly defined by their cis/trans microstructure and supramolecular ordering. PTMGP with well-defined microstructure (trans content between 80 and 90%) can be considered as potential membrane material for gas separation, e.g., separation of higher hydrocarbons from natural gas. PTMGP/TiO2 nanocomposite membranes were also studied for exploring the gas permeability properties and stability of polyacetylene membranes. Gas transport properties were affected slightly by TiO2 presence; however, the long-term stability was enhanced.