Poly(4-methyl-2-pentyne) [PMP] is an amorphous, glassy, di-substituted acetylene-based polymer. PMP has a low density of 0.78 g/cm(3) and a high fractional free volume of 0.28. The permeabilities for helium, hydrogen, nitrogen, oxygen, carbon dioxide, methane, ethane, propane, and n-butane were determined at temperatures from 20 to 65 degrees C and pressures from 10 to 150 psig. PMP is the most permeable purely hydrocarbon-based polymer known; its permeabilities are only exceeded by poly(1-trimethylsilyl-1-propyne) [PTMSP] and poly(1-trimethylgermyl-1-propyne) [PTMGeP]. The oxygen permeability of PMP at 25 degrees C is 2700 x 10(-10) cm(3)(STP) cm/cm(2) s cmHg and the nitrogen permeability is 1330 X 10(-10) cm(3)(STP) cm/cm(2) s cmHg. The high gas permeabilities in PMP result from its very high free volume, and probably, interconnectivity of the free-volume-elements. For a glassy polymer, PMP exhibits unusual organic vapor permeation properties. Permeabilities in PMP are higher for large, condensable gases, such as n-butane, than for small, permanent gases such as helium. The permeabilities of condensable gases and permanent gases decrease as the temperature is increased. This behavior is completely unexpected for a glassy polymer and has been observed previously in only high-free-volume glassy PTMSP.