A sulfonic acid-functionalized trimethyl-substituted polyimide was synthesized by reacting 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,5-diamino-2,4,6-trimethylbenzenesulfonic acid (TrMSA). The properties of 6FDA-TrMSA were compared to the related 6FDA-derived polyimide analogues made from 2,4,6-trimethylbenzene-1,3-diamine (6FDA-TrMPD) and 3,5-diamino-2,4,6-trimethylbenzene benzoic acid (6FDA-TrMCA). Compared to 6FDA-TrMPD and 6FDA-TrMCA, sulfonic acid functionalization resulted in significantly lower surface area, reduced fractional free volume, and tighter chain d-spacing. Consequently, 6FDA-TrMSA displayed lower gas permeabilities with a commensurate increase in permeability-based gas-pair selectivities. The enhanced CO2/CH4 selectivity of 6FDA-TrMSA was caused exclusively by higher diffusion selectivity, which was promoted by strong hydrogen bonding induced by the [sbnd]SO3H functionalization. Permeation experiments of 6FDA-TrMSA with a 1:1 CO2-CH4 mixture revealed the occurrence of competitive sorption effects (depressing CO2 gas permeability) and CO2-induced polymer matrix plasticization, which reduced the polymer selectivity by enhancing CH4 permeability. At ~20 atm total pressure, 6FDA-TrMSA showed a CO2 permeability of ~15 Barrer and an equimolar CO2/CH4 mixed-gas selectivity of 55, which are ~2-fold higher performance values than those of the state-of-the-art polymer used for industrial scale natural gas sweetening, i.e., cellulose triacetate.