The pure-gas permeation properties for a systematic series of disubstituted, amorphous, glassy poly(2-alkylacetylenes), i.e., poly(2-hexyne), poly(2-octyne), poly(2-nonyne), poly(2-decyne), and poly(2-undecyne), were determined at 35 degreesC for H-2, CO2, O-2, N-2, CH4, C2H6, C3H8, and n-C4H10. Gas permeability increased by increasing the (i) side-chain length of the polymers and (ii) size and condensability of the penetrants. For example, the nitrogen and n-butane permeabilities of poly(2-undecyne) were 2- and 10-fold higher than those of poly(2-hexyne). As the fractional free volume decreases with increased side-chain length, the permeability increases. This behavior is opposite to that observed in polyacetylenes containing terminal bulky side-group substituents, such as trimethylsilyl or isopropyl groups. In branched acetylene-based polymers, the gas permeation properties correlate well with their free volume values; as fractional free volume increases, a simultaneous increase in permeability is observed. In linear poly(2-alkylacetylenes), an increase in side-chain length increases the overall chain mobility and increases the chain packing density of the polymers. In mixed-gas permeation experiments, an increase in mixed-gas n-butane permeability and n-butane/methane selectivity was observed with increasing side-chain length of poly(2-alkylacetylenes).