Systematic studies on gas permeation of pure SF6 and N2 as well as their mixture in poly(4-methyl-1-pentene) (PMP) at different temperatures and pressures, using commercially available thin PMP films, are reported in this article. The effective separation of SF6 from binary mixtures with N2 is critical for the proposed replacement of pure SF6, used as an insulating gas in high power industry, by the mixtures of these two gases. This replacement is driven by the fact that SF6 is the most potent greenhouse gas, with a global warming potential of 22,200 times that of CO2. The experiments with a 1:1 mixture of N2 and SF6 revealed the permselectivity of PMP as high as 476 with the corresponding N2 permeability coefficient of 7.6 Barrer. These properties, which are much better than those of other glassy polymers considered for this separation, were not affected by a long-term exposure to SF6, which indicates the excellent resistance of PMP to plasticization by this gas. Using a single stage membrane system utilizing the PMP membrane would allow separating the above gas mixture into a 99% pure SF6 product with the corresponding recovery rate of SF6 greater than 99%.